Heat oven to 400 degrees. Cut frozen fish fillets diagonally into halves if desired. (For easier cutting let fish stand at room temperature for 5 to 10 minutes.)

Heat margarine over low heat until melted; stir in flour. Cook over low heat, stirring constantly, until mixture is smooth and bubbly. Remove from heat; stir in milk, onion and garlic salt.

Heat to boiling, stirring constantly. Boil and stir for 1 minute.

Spread spinach evenly in ungreased rectangular baking dish(11 x 7 x 1 1/2 inches); sprinkle with lemon juice. Spoon sauce evenly over spinach; sprinkle with cheese. Arrange fish, overlapping slightly, in 2 rows on spinach.

Bake uncovered until sauce is bubbly and top of fish is crisp, 18 to 20 minutes. Sprinkle with paprika if desired.

Makes 3 to 5 servings. Preparation time: 35 minutes.