To retain flavor and nutrients, try to stir-fry vegetables and serve them immediately. Avoid prolonged heating or reheating. For tougher stems, add a small amount of water after a quick stir-fry and let them simmer.
To create a deliciously thick and rich sauce when cooking vegetables, add some starch. This will also help retain the vitamins in your dish, as starch protects them during cooking.
When cooking bean sprouts or other vegetables, add a bit of vinegar. This enhances the flavor and preserves the vitamins, as vinegar helps protect nutrients.