Preheatoven to 350. Lightly grease a 13x9 inch baking dish.
In a large stockpot, combine squash, bell peppers, onion, garlic, and 1 tablespoon salt; add water to cover. Bringto a boil over medium-high heat; reduce heat, and simmer for 6 minutes oruntil tender. Drain well.
In a large bowl, combine squash mixture, 3 cups cheese,2cups crushed crackers, sour cream, egg, thyme, pepper, and remaining 1 teaspoon salt. Spoon into prepared baking dish; sprinkle with remaining 1 cup cheese.
In a small bowl, stir togther melted butter and remaing 1 cup crushed crackers;sprinkle over caserole. Bake for 30 minutes or until hot and bubbly. Serve immediately