What is a main ingredient in ratatouille?

Easy...It's actually fairly simple and there are dozens of variations. It's one of those foods you really can't get "wrong".

Basically, you slowly simmer tomatoes, eggplant, onions, red/green bell peppers, zucchini, garlic and herbs in olive oil. Salt and pepper to taste, and use whatever balance of the above vegetables that you prefer. I like to add mushrooms instead of eggplant, which some people don't like due to its strong taste. Some people add some tomato sauce/paste as well, to give it something to simmer in. If you do that, saute everything briefly first and then add the sauce.

Ratatoullie is good served cold, room temp, warm or hot.