Preheat oven to 375°.
Heat oil in a large nonstick skillet over medium-high heat. Add onion; sauté 15 minutes or until golden brown.
Cut 2 bread slices into 1-inch cubes; set aside. Drain tomatoes in a colander over a bowl, reserving 1/2 cup liquid; discard remaining liquid. Place tomatoes in a medium bowl. Stir in basil, thyme, pepper, and garlic.
Place 1/2 cup onion in the bottom of a 3-quart casserole dish coated with cooking spray. Arrange 2 bread slices, half of the remaining onion, half of the tomato mixture, half of the zucchini, and half of the cheese over onion in dish. Repeat layer; top with bread cubes. Pour 1/2 cup reserved tomato liquid and broth over casserole. Cover and bake at 375° for 30 minutes. Uncover casserole and bake an additional 25 minutes or until top begins to brown.
Spoon 2 cups casserole into each of 4 shallow bowls; top each serving with 1 tablespoon Red-Pepper Aioli.
Totals include Red-Pepper Aioli.