There is a reason chefs keep anchovy paste, parmesan, tomato paste, mushrooms, miso or dried porcini close at hand. These ingredients add umami, that deeply savoury quality that makes food taste more complete and satisfying. Umami is not a single flavour so much as a sense of fullness. It gives dishes weight without making them heavy.
A little tomato paste cooked down in oil can transform a sauce. Parmesan rind dropped into soup can create remarkable depth. Mushrooms, dried or fresh, add an earthy, brooding complexity. These ingredients are often used sparingly, but their effect is rarely small.