Preheat the oven to 375 F
Slice the onion, eggplant, tomatoes and zucchini into thin slices
Mince the garlic, sauté the garlic with a little bit of olive oil in a skillet pan
Add in tomato sauce and cook for a few minutes
Pour half of the Sauce into the baking dish
Stack the veggie slices in alternating patters and place them in a baking pan
Repeat until the pan is filled and all the veggie slices are used up
Pour the rest of the tomato sauce over it
Sprinkle the exposed tops of the veggies with oil , salt and pepper
Cover with aluminium foil
Bake for about 45 minutes, until veggies are tender