Zucchini and Summer Squash Gratin - can be made Vegan!

It's that over abundance of zucchini and Summer squash time of year.  This is a great side dish to use some of that veggie!  Hearty and tasty!  The recipe below is my full recipe, but the pictures show it cut in half.To make this you will need:
2 Tbs. olive oil

2 medium onions (14 oz. total), thinly sliced

2 cloves garlic, minced 

8 plum tomatoes, cut in 1/4 inch slices

2 zucchini cut in 1/4 inch slices

2 yellow summer squash, cut in 1/4 inch slices

3 tablespoons olive oil

1/4 cup fresh thyme leaves

1 teaspoon salt

1/3 cup grated parmesan cheese (Vegan - Omit)

1 sleeve Ritz crackers, ground

1 tablespoon thyme leaves

1.)  In a medium skillet, heat the olive oil over medium heat. Add the onions and sauté, stirring frequently, until limp and golden brown, about 20 minutes. Reduce the heat to medium-low if they're browning too quickly. Add the garlic and sauté until soft and fragrant, 1 to 2 minutes Spread the onions and garlic evenly in the bottom of an oiled 2-qt. shallow gratin dish.
2.)  Heat the oven to 375°F. Put the tomato slices on a shallow plate to drain for a few minutes and then discard the collected juices. In a medium bowl, toss the zucchini and squash slices with 1-1/2 Tbs. of the olive oil, the thyme, and 1/2 tsp. of the salt.  Starting at one end of the baking dish, lay a row of slightly overlapping tomato slices across the width of the dish. Next, lay a row of zucchini, overlapping the tomatoes by two-thirds, and sprinkle with cheese. Repeat with a row of squash, and then repeat rows, until the gratin is full.
Season lightly with pepper and the remaining 1/2 tsp. salt. Drizzle the remaining 1-1/2 Tbs. olive oil over all. Combine the cheese with the remaining 1 Tbs. thyme and the cracker crumbs.  Sprinkle this over the whole gratin. Cook until well-browned all over and the juices have bubbled for a while and reduced considerably, 65 to 70 minutes. Let cool for at least 15 min. before serving.