Beat egg slightly; set aside 2 tablespoons. Stir tuna, onion, cheese, parsley, celery ... remaining egg. Stir Bisquick and water to soft dough; ... in cheese until melted.
Heat oven to 450 degrees. Mix water and Bisquick, beat vigorously for 20 ... mayonnaise, horseradish, and tuna. Spread in baked shell. ... 2 hours or overnight.