Preheat oven to 350°F. Meanwhile, coat two 9- by 13-inch casserole dishes with cooking spray and set aside. In a large stockpot over medium heat, combine the squash, yellow onions, and bell pepper. Cover vegetables with water, bring to a boil, and cook until squash is just soft, 10 minutes. Drain, discarding the liquid, and return vegetables to the pot. Add the next 12 ingredients (Cheddar through sugar). Season with salt and freshly ground pepper, to taste. Stir to mix well. Scoop mixture into the prepared casserole dishes, and sprinkle with breadcrumbs. Bake until golden brown, 30 to 45 minutes.