Preheat oven to 350 degrees. Cut slits around the perimeter of the whole spaghetti squash. Place on a plate, and microwave for 5 minutes, or until warm and slightly tender. Slice in half lengthwise, and place on a baking sheet.

Bake for 30 minutes. Remove from heat and set aside. Change the oven temperature to 375.

Warm the olive oil in a saute pan. Add the onion and garlic, and cook for 2 minutes. Add in the bell pepper, zucchini squash, salt, pepper and Italian seasoning. Cook until mostly tender, around 7 minutes.

Spoon a thin layer of marinara sauce (about half) into an 8x11 baking dish. Comb out the strands of half of the spaghetti squash, discarding the slimy stuff and seeds. Place the edible parts in the baking sheet on top of the marinara.

Spoon half of the sautéed veggies on top of the spaghetti squash in the dish, then spread 1/2 of the cheese on top.

Repeat the last two steps once more, layering the casserole with sauce, spaghetti squash, sautéed veggies & cheese.

Sprinkle the grated parmesan cheese on top of the casserole. Cover with foil, and bake for 15 minutes.

Remove the foil, and cook again until the cheese looks bubbly and lightly browned, around another 10 minutes.

Serve the casserole with fresh bread for dipping!