Preheat oven to 350 degrees F (175 degrees C).
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, ¼ cup butter, parsley and garlic salt. Stir until the mixture is lukewarm and blended. Pour sauce over the enchiladas, and sprinkle with remaining cheese.
Bake in preheated oven for 30 minutes.
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