1. Put butter, onion and garlic into a 1 1/2-quart glass casserole. Cook uncovered in microwave oven 4 minutes at HIGH, or until onion is tender; stir after 1 minute.
2. Add soup, milk, cooked shrimp and peas; mix. Cover with waxed paper. Cook 4 to 5 minutes at HIGH, or until thoroughly heated; stir occasionally.
3. Serve shrimp mixture in patty shells.