In a small saucepan, saute onion in 1 tablespoon butter until tender. Reduce heat to low. Add cream cheese and sour cream; cook and stir until smooth. Add turkey; cook until heated through (do not boil).
Spoon 1/2 cup filling onto each of four bread slices; top with a slice of cheese. Spread outside of bread with remaining butter. In a large skillet over medium heat, cook sandwiches until lightly browned on both sides.