Heat oven to 400 deg. Grease 9x9 inch pan. Mix Bisquick, milk, eggs, salt and pepper. Beat vigorously 30 sec. Stir in 1.5 cans tuna, 0.25c cheese, onion, pimiento and celery. Pour into prepared pan; sprinkle remaining 0.25c cheese over top. Bake 30-35 mins. Serve hot with Tuna Celery Sauce. Tuna Celery Sauce: In small saucepan,mix soup, milk and remaining tuna. Heat through.