Preheat oven to 350 degrees F (175 degrees C).
In a large pot of salted water, boil squash and onions until tender.
In a saucepan, combine butter or margarine and cream; stir constantly over a low heat. Mix in cheese, cornstarch, salt and pepper. Stir until sauce thickens.
Place squash and onions in a 2-quart baking dish. Mix in shrimp and cheese sauce. Sprinkle casserole with cracker crumbs.
Bake uncovered for 20 to 30 minutes or until hot.