Drain salmon; flake.
Combine celery and onion in butter in saucepan; saute for 5 minutes.
Blend in flour, salt and pepper; add milk gradually.
Cook, stirring constantly, until thickened and smooth.
Stir in salmon, rice, mushrooms and cheese; turn into casserole.
Bake at 350 degrees for 20 minutes or until bubbly and heated through.
Garnish with parsley.