Preheat the oven or grill to 375.
Peel and thinly slice 1 onion. Saute in 2 T. olive oil in a large cast iron skillet over low heat with 1 t. fresh thyme leaves or a pinch of dried thyme.
Meanwhile, peel and core 4 sweet peppers. Slice lengthwise, then cut the slices in half. When the onions are soft, add the peppers to the pan. Season with salt and pepper.
Cut 2 Delicata Squash in half lengthwise and scoop out the seeds and strings. Slice the squash in half-rounds. Toss in a large bowl with 1 t. salt and 1 T. olive oil.
Remove the pan from the heat and arrange the Delicata slices in a single layer atop the onion/pepper mixture.
Minced 2 cloves of garlic and sprinkle over the squash.
Thinly slice 2-3 tomatoes and arrange the slices over the squash. Sprinkle with more fresh or dried thyme and drizzle another 1 T. of olive oil over them.
Slice mozzarella cheese and arrange the cheese over the tomatoes.
Cover the pan and bake at 375 degrees in the oven, or on the gas grill, for 20 minutes. When the squash are tender, remove the foil to brown the top of the gratin.