In a large skillet, saute onions in 1 tablespoon butter until transparent. Remove the skillet from heat and stir in crabmeat and shrimp. Shred the cheese and mix half of it into the seafood. Place a large spoonful of the mixture into each tortilla. Roll the tortillas up around the mixture and arrange the rolled tortillas in a 9x13 inch baking dish.
In a saucepan over medium-low heat, combine half-and-half, sour cream, 1/4 cup butter, parsley and garlic salt. Stir until the mixture is lukewarm and blended. Pour sauce over the enchiladas, and sprinkle with remaining cheese.