Grease a casserole dish and preheat oven to 350 degrees.
Melt butter in a large pan over medium heat. Add asparagus, mushrooms, and onions and cook, stirring occasionally, for about 6 minutes.
Sprinkle flour over vegetables and cook, stirring continuously, for 1 to 2 minutes.
Gradually stir in milk. Cook until thickened.
Reduce heat to low and mix in mayonnaise, sour cream, both cheeses, salt, lemon pepper, and paprika. Remove from heat.
Crush about 1/3 of the can of french-fried onions and mix them into the asparagus mixture.
Transfer mixture to prepared casserole dish.
Sprinkle remaining french-fried onions on top.
Place in oven and bake for 30 minutes.