Seaside Shrimp Casserole

1 lb fish, such as cod or sole
1 medium onion, thinly sliced
2 cloves garlic, minced
1 tablespoon butter or margarine
1/2 tsp. salt
Dash ground pepper
Fresh lemon juice to taste
1 cup sour cream
1 tsp. dijon style mustard
1 1/2 cup cooked medium shrimp, shelled and deveined
Dill or parsley, chopped

Wipe fish with a damp cloth. Cut into serving pieces, or cubes. Saute onion in butter until soft but not browned. Add garlic and fish. Heat to simmer. Cook for 10 to 20 minutesdepending on type of fish used and size of pieces. Fish should be just tender and flake easily. Don't overcook. Salt and pepper to taste. Sprinkle with a little lemon juice. Mix sour cream and mustard. Just before serving, add sour cream mixture and shrimp to casserole. Heat until just hot without stirring. Top with dill or parsley.