There are now a number of healthy foods which research has found can improve endothelial function and reverse endothelial dysfunction.
Endothelial dysfunction is a systemic pathological state of the endothelium (the inner lining of blood vessels) and can broadly be defined as an imbalance between vasodilating and vasoconstricting substances produced by (or acting on) the endothelium.
Research has found that endothelial dysfunction can now be considered an early predictor for atherosclerosis and cardiovascular disease risk.
Some examples of foods which have been shown to improve endothelial function include walnuts, dark chocolate, green tea, apples, nitrate rich plant-foods such as spinach, amongst many others which we will be discussing below.
Walnuts appear to have additional and superior heart-health benefits over other tree-nuts such as cashews, pistachios and almonds.
One of the benefits that research has uncovered is walnuts ability to improve endothelial function, amongst improving numerous other traditional cardiovascular disease risk factors such as lowering LDL/total cholesterol levels, reducing inflammatory and oxidative stress biomarkers.
Walnut enriched diets have been shown to improve endothelial function in many different types of test subjects including individuals with Type 2 Diabetes and hypercholesterolemia.
Compared with other tree-nuts, Walnuts are an excellent plant-based source of the n-3 polyunsaturated fatty acid (PUFA)/omega-3(ALA), which may be partly responsible for the additional cardioprotective effects.
Omega-3 fatty acids can trigger activating pathways in endothelial cells promoting an increased formation of nitric oxide and endothelium-dependent hyperpolarization.
2. Dark Chocolate & Cocoa Flavanols
Evidence suggests that high-polyphenol dark chocolate(70% cocoa+ content) can support cardiovascular health through a variety of mechanisms including lowering blood pressure and improving endothelial function.
Dark chocolate and cocoa is a rich source of antioxidants, particular flavonoids and flavanols such as epicatechin and catechin.
Cocoa is also a good source of the mineral magnesium, which is also extremely important for maintaining healthy cardiovascular function and blood pressure.
A meta-analysis from 2013 found:
Pooled estimate showed that intervention with dark chocolate significantly increased FMD levels by 2 % (95 % confidence interval 1.6-2.39 %) compared with placebo/control group.
In summary, intervention with cocoa improved endothelial function as measured by FMD. 
Generally speaking the higher the cocoa content, the more flavanols the chocolate tends to contain and less refined sugar.
3. Olive Oil
A number of clinical trials have found that extra virgin olive oil may improve endothelial function, amongst various other cardiovascular related benefits.
Olive oil is a component of the traditional Mediterranean diet and has a good body of research supporting its heart-health benefits and ability to reduce cardiovascular disease risk.
Olive oil bioactive compounds exhibited a potent capability to attenuate oxidative stress and improve endothelial function through their anti-inflammatory, anti-oxidant, and anti-thrombotic properties, therefore reducing the risk and progression of atherosclerosis. 
A systematic review and meta-analysis published in 2015 found:
Values of flow-mediated dilatation (given as absolute percentage) were significantly more increased in individuals subjected to olive oil interventions (mean difference: 0.76% (95% CI 0.27 to 1.24), p < 0.002, n = 8 trials).
These results provide evidence that olive oil might exert beneficial effects on endothelial function as well as markers of inflammation and endothelial function, thus representing a key ingredient contributing to the cardiovascular-protective effects of a Mediterranean diet. 
4. Red Wine & Grapes
Red wine and grapes have long been associated with supporting cardiovascular health.
Red grapes are an excellent source of many bioactive nutrients and phyto-chemicals such as polyphenols, resveratrol, anthocyanins, flavanols and many others.
Studies have found acute intake of red wine and red grape improves endothelial-dependent vasodilatation.
Polyphenolic compounds from red grapes acutely improve endothelial function in patients with coronary heart disease. 
Endothelial function can be significantly improved in healthy adults in the initial 2 h after intake of grape polyphenols. 
Berries are another good choice of fruit to incorporate as part of a heart-healthy diet and are a great source of polyphenols including anthocyanins, flavan-3-ols, procyanidins, flavonols, ellagitannins and many others.
Research has found that berry intake can improve a number of CVD risk factors and reduces TC(total cholesterol), LDL, TG(triglycerides), and BP(blood pressure) while increasing the level of HDL cholesterol.
Studies have shown that berries such as blueberries and strawberries may also improve endothelial function and exert a beneficial impact on vascular function.
Current evidence suggests that berries, at physiological relevant doses, may have a role in the modulation of vascular function and stiffness. 
Daily intake of strawberries may improve endothelial function and acute changes in blood pressure, independent of other metabolic changes, and may be considered a specific food/fruit to include in a heart-healthy diet in overweight or obese subjects with moderate hypercholesterolemia. 
6. Black & Green Tea
Both black and green tea of the Camellia Sinensis variety have been shown to improve endothelial function in individuals with coronary artery disease and healthy smokers.
Tea is an excellent source of antioxidant flavonoids such as catechins, which is the active constituent thought to be responsible for the endothelial and cardiovascular related benefits associated with tea consumption.
The number of catechins in non-fermented tea (green tea) is typically higher than that in fermented tea (black tea).
The study concluded that short and long-term black tea consumption reverses endothelial vasomotor dysfunction in patients with coronary artery disease. 
The study concluded that both green and black tea are equally effective in improving endothelial function. 
The results of the study suggest that green tea consumption reverses endothelial dysfunction in healthy smokers, possibly through its antioxidant effect. 
7. Nitrate-Rich Leafy Greens Such as Spinach
Flavonoids aren’t the only phyto-nutrient shown to improve endothelial and vascular function in studies.
Research has found that nitrate rich leafy greens such as spinach may also improve endothelial function and augment NO(nitric oxide) bioavailability via NO-pathways.
Dietary nitrate has been demonstrated to have a range of beneficial vascular effects, including reducing blood pressure, inhibiting platelet aggregation, preserving or improving endothelial dysfunction, enhancing exercise performance in healthy individuals and patients with peripheral arterial disease. 
In conclusion, flavonoid-rich apples and nitrate-rich spinach can independently augment nitric oxide status, enhance endothelial function, and lower blood pressure acutely, outcomes that may benefit cardiovascular health. 
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.
Always consult with a qualified healthcare professional before making any significant dietary or lifestyle changes including supplements and herbs.