Preheat the oven to 400ºF. Dice the onion and mince the garlic. Sauté the onion and garlic with olive oil in a large skillet over medium heat until the onions are soft and transparent (about 5 minutes).

While the onion and garlic are sautéing, slice the zucchini, yellow squash, tomato, and potato thinly (1/4-inch slices). Make sure the potatoes are very thin so that they soften quickly while in the oven.

Coat the inside of an 8x8" casserole dish with non-stick spray. Spread the sautéed onion and garlic in the bottom of the dish. Arrange the other sliced vegetables, stacked vertically like dominos, in an alternating pattern. Season the top of the vegetables generously with salt, pepper, and dried thyme.

Cover the dish with foil and bake in the preheated oven for 30 minutes. Remove the foil, top with the shredded cheese, and bake without the foil for an additional 15-20 minutes, or until the cheese is golden brown. Top with chopped fresh parsley for garnish, if desired.