Add the garlic and cook for another minute. Spread the onion mixture on the bottom of the baking dish. Slice the potatoes, zucchini, and tomatoes in 1/4-inch-thick slices. Place them alternately in the dish on top of the onions, as tight as possible in a single layer. Sprinkle with salt, pepper, thyme and drizzle with 2 more tablespoons of olive oil.