Sautee onions, garlic, and bell peppers in olive oil, stirring frequently, for 20 minutes. Add more olive oil as necessary to prevent burning.
Peel and core the tomatoes, and add to the pot. Simmer, covered, 15 minutes. Stir harshly enough to break cooked tomatoes into pieces.
Add eggplant and zucchini, Herbes de Provence, and salt and pepper. If your tomatoes are off-season or otherwise less than perfect, add up to 1/2 cup tomato paste to improve flavor. Recover and simmer 30 minutes.
This is an easy, one-pot version of a traditional French ratatouille. Serve hot or cold, as a soup or over rice or couscous.