Make Sauce: Melt butter. Saute onion until transparent. Blend in flour, stirring constantly. Slowly stir milk into saucepan mixture. Cook until smooth and thick, stirring constantly. Add lemon juice and parsley. Mix well.
Spoon pasta into sprayed casserole dish. Put salmon on top. Sprinkle with 3/4 cup cheese. Pour sauce over it, and sprinkle with the rest of the cheese.
Bake at 375 for 25 min or until hot and lightly browned.
NOTE: This doesn't make for a lot of sauce. To keep original proportions, double the butter, onion, flour, milk, lemon juice and parsley.