Cook broccoli according to package directions; drain. In 4 individual au gratin dishes arrange broccoli and crab-flavored fish pieces. Set dishes aside.
For sauce, in medium saucepan cook mushrooms and garlic in margarine over medium-high heat about 4 minutes or until mushrooms are tender. Stir in flour and pepper. Then, stir in milk. Cook and stir until thickened and bubbly; remove from heat. Stir in cheddar cheese until melted. Spoon sauce on top of broccoli and fish in dishes. Cover dishes with foil and chill for 2 to 24 hours.
Before serving, bake, covered, in a 400 degree F. oven for 20 to 25 minutes or until bubbly. Combine Parmesan cheese and cracker crumbs and sprinkle on top of each serving. Makes 4 servings.