Preheat the oven to 450 degrees. Sprinkle the eggplants with salt, put in a colander and let stand for 10 minutes. Rinse with cold water and pat dry. Pour the olive oil into a 12 inch glass baking dish. Heat the oil in the oven for 5 minutes and remove.
Arrange the potato and onion slices in the baking dish and bake for 30 minutes or until the potatoes are slightly tender. Arrange the zucchini and eggplant slices on top of the potatoes and bake for 5 minutes. Sprinkle the red peppers over the other vegetables. Beat the eggs and season with salt and pepper.
Pour the eggs over the vegetables and arrange the thyme on top. Bake until the eggs are set and the sides have puffed, about 20 minutes. (Reduce oven temperature if eggs cook too fast.) The top will become golden brown and a knife inserted in the middle should come out clean. Do not overcook. The frittata should be firm but not dry. Serve hot or at room temperature.
