Grease 13 x 9-inch baking dish. Wash and peel squash. Slice lengthwise (about 1/4-inch thick) and place overlapping layers in dish, using half of squash. Brown meat; drain well. Add tomato sauce, 1/2 teaspoon salt, and pepper to meat; simmer for 2 minutes. Spread this mixture over squash. Sprinkle with 1 cup mozzarella cheese. Place another layer of squash over meat and cheese. Sprinkle squash with remaining 1 teaspoon salt. Prepare cream sauce and pour over squash. Bake at 350 degrees for 1 hour or until squash tests tender with fork and top is lightly browned.
Sauce
Melt margarine; stir in flour. Remove from heat and slowly add milk. Return to heat and bring to boil, stirring until thick. Add cheese. When mixture is smooth and creamy, spread evenly over top of squash.
HINT: This casserole may be baked for 1/2 hour, cooled to room temperature, and frozen.