Cook pasta. Meanwhile in a large skillet saute mushrooms and onions in 1 T butter until tender.
In a large bowl beat ricotta and 1/2 cream cheese until blended. Stir 1/2 c Asiago cheese, Parmesan cheese, bacon, 1 T parsley, garlic salt, nutmeg, pepper and mushroom mixture. Spoon into shells, place in a greased 9x13 baking dish.
In a large saucepan melt remaining butter. Stir in flour until smooth, gradually adding the cream, broth and milk. Bring to a boil, cook and stir for 1-2 minutes or until thickened.
Stir in Romano cheese and the remaining cream cheese, Asiago and parsley. Pour over shells. Sprinkle with mozzarella cheese. Cover and bake at 350 for 30 minutes. Uncover and bake 10-15 minutes longer or until bubbly.