Heat the olive oil in a large frying pan, add onion slices and sauté until they are transparent. Add the diced green pepper and aubergines and, 5 minutes later, the baby marrows and tomatoes. The vegetables should not be fried but stewed in the oil, so simmer gently in a covered pan for 30 minutes. Add salt and pepper to taste, chopped parsley, marjoram, basil and crushed garlic; then cook, uncovered, for about 10 to 15 minutes, or until ratatouille is well mixed and has the appearance of a ragoût of vegetables—which it is. Serve hot from the casserole, or cold as a delicious beginning to a summer meal.