From the “All Recipes” Web site

To help your vegetables reach their full flavor potential, sauté or roast them before you add them to your soup pot. This process caramelizes the naturally occurring sugars in the vegetables, giving them a much richer, deeper flavor.

To caramelize your vegetables, chop them into the size you desire for your soup, then, over a medium-high burner, heat a nonstick pan that’s just the right size to contain your vegetables in a single layer (this is important-your vegetables will steam instead of caramelize if they’re stacked on top of each other), pour in a small amount of oil or butter, and add your vegetables. Stir or toss them often enough to keep them from burning, but be patient. Good browning can take some time.
Another way to add flavor to your vegetables is by pan-roasting them in a hot oven (450 F/230 C). Toss them very lightly in oil, place them on a sturdy baking sheet in a single layer, and do not cover them. Stir occasionally, and remove when golden-brown.