Melt butter and saute onions and celery until tender. Add flour and milk. Stir until smooth. Blend in rice and seasoning. Pour into a 12 x 7-1/2 x 2 inch baking dish. Dip oysters in melted butter, roll in crumbs and arrange on top of rice. Place under broiler 10 minutes until edges begin to curl.
Meanwhile, make a sauce by sauteing mushrooms, grated onion and butter about 5 minutes. Stir in flour, salt and pepper. Cook until smooth and bubbling. Add milk; bring to boil. Cook 1 minute, stirring until thickened. Pour over rice and serve.
Cajun Country -- New Iberia, Louisiana