Preheat your oven to 400F and have a glass or ceramic baking dish ready. A pie plate or a 9"x9" dish works best.
Make the Olive Oil Mixture by combining the oil, onion, garlic, the 3 tablespoon of basil, and the salt in a small bowl. Mix until well combined and set aside.
Slice the tomatoes and the zucchini in ¼ inch slices. Depending on the size of the tomatoes, the quantity of tomatoes needed can vary.
Lightly grease the baking dish and layer the sliced veggies. Alternate between the tomatoes and zucchini to fill the dish.
Spoon the olive oil mixture evenly over the overlapping tomatoes and zucchini.
Sprinkle the veggies with the grated mozzarella and place the dish (uncovered) in the preheated 400F oven. Bake for 25-30 minutes.
Remove the dish from the oven and drizzle the casserole with the balsamic vinegar and sprinkle on the 1 tablespoon of finely chopped basil.