In a small saucepan, bring water to a boil. Gradually stir in grits. Reduce heat; cover and simmer for 4 minutes. Stir in the cheeses, 1 tablespoon butter, garlic salt, salt and pepper.
In a large skillet, saute shrimp and onions in remaining butter until shrimp turn pink. Fill tart shells with grits; top with shrimp mixture. Refrigerate leftovers.