Combine apple, celery, and butter or margarine in a 1-quart microwave-safe bowl.
Cover the bowl and microwave on 100% power (HIGH) for 2-3 minutes, or until the celery reaches a crisp-tender texture.
Add stuffing mix, mayonnaise or salad dressing, walnuts, and lemon juice to the bowl; mix well and set aside.
In a 12 x 7 1/2 x 2-inch baking dish, place the block of fish.
Cover the dish and microwave for 5 minutes, giving it a half-turn after 3 minutes.
Drain off any excess juices.
Create 4 mounds of stuffing around the fish in the dish.
Cook the dish uncovered for 2-3 minutes or until the fish easily flakes when tested with a fork, and the stuffing is heated through.