Place fish on a baking sheet and bake for 15 minutes or until well browned and fish flakes easily.
Meanwhile, cook spinach according to package directions, adding celery at the beginning of cooking. Cook until celery is crisp-tender. Press spinach-celery mixture against the sides of a sieve with a rubber scraper to remove excess moisture. Return spinach to saucepan.
While spinach is cooking, prepare sauce with 1 cup milk according to package directions; stir in dill weed and pepper.
Stir 1/2 cup of the white sauce into cooked spinach. Spread spinach mixture evenly in an ungreased shallow 1-quart casserole. Top with fish. Spoon reserved sauce over fish without completely covering fish. Top with cheese in a decorative pattern and sprinkle with bacon bits. Bake for 3 to 5 minutes or until cheese melts.