1. Melt 3 tablespoons butter in a 1-quart glass measuring pitcher in microwave oven (about 1 minute at High). Stir in flour and 1/2 teaspoon salt. Add milk and wine gradually, stirring until smooth. Cook uncovered 4 minutes at High, or until thickened; stir once. Stir in cheese; set aside.
2. Put onion and 2 tablespoons butter into a 1-quart glass casserole. Cook uncovered in microwave oven 2 minutes at High, or until onion is tender; stir after 1 minute. Add crab, mushrooms and crumbs to onion; mix well. Season with salt and pepper.
3. Cut fillets into uniform serving-size pieces. Lay each fillet white-side-down and sprinkle with salt and pepper. Dividing the stuffing evenly, spread stuffing almost the full length of each fillet. Roll up and place seam-side-down in an 8-inch square glass baking dish. Cover with waxed paper.
4. Cook in microwave oven 5 minutes at High; rotate dish one-half turn after 3 minutes and rearrange fish. With a bulb baster, draw off any juices in the baking dish and stir into the sauce. Cover and cook 4 to
5 minutes at High, or until fish flakes easily when tested with a fork. Remove from oven. Cover with plastic wrap and let stand 5 minutes.
5. Meanwhile, heat sauce uncovered in microwave oven 5 minutes at High, or until cheese is melted; stir once. Pour sauce into a sauce boat and garnish with parsley.