Quarter the eggplant and cut away the flesh to within 1/2 inch (1.3 cm) of the skin. Save the center of the eggplant for another use - a soup perhaps - and cut the skin in slivers about 1/4 inch (6 mm) wide.
Heat the oil in a large, heavy skillet.
Add the onion and garlic and saute over medium heat, stirring, until the onion is golden, about 15 minutes.
Add the peppers and saute another 10 to 15 minutes.
Add the eggplant and zucchini and, if needed, more oil. Continue to saute another 15 to 20 minutes.
Stir in the tomatoes and cook another few minutes, then season with salt and pepper, stir in the basil, and serve. Or set aside and reheat to serve with other ingredients.