Melt 2 tablespoons of the butter in a saucepan. Add mushrooms, celery, onion, green pepper and basil. Saute the Mushroom Mixture for about 10 minutes or until celery is tender.
While mushrooms are sauteing, make the White Sauce by melting 4 tablespoons of butter in the top of a double boiler. Stirring constantly, gradually add the flour. Add salt, pepper and nutmeg. Slowly pour in milk. Cook and stir for about 6 minutes until sauce is thick and smooth.
Add stingray to the Mushroom Mixture and cook for about 5 minutes. Combine stingray and Mushroom Mixture with the White Sauce and stir well. Pour everything into a casserole dish. Cover with cracker crumbs and top with grated cheddar cheese.
Bake in oven for approximately 20 minutes or until casserole is golden brown.