Preheat oven to 350F. Wash, cut and halve eggplants and zucchini and one bell pepper. Using a spoon hollow out eggplants and zucchini to create a boat;transfer them to baking dish. Roughly chop scooped-out eggplant and zucchini flesh,mushrooms,one bell pepper.In a large skillet over medium heat, heat oil. Add onion and cook until soft, 5 minutes. Stir in chopped eggplant and zucchini and season w salt and pepper