Preheat oven to 400 degrees F. Add flour to a Reynolds(R) Large Oven Bag. Shake. Place bag in 2-quart rectangular baking dish at least 2 inches deep.
Place spinach in bag; sprinkle with onion. Top with fish. Stir together the butter, garlic, salt, and black pepper. Brush over fish. Arrange lemon slices over fish. Close bag with tie (found inside package). Cut six 1/2-inch slits in top of bag to allow steam to escape. Tuck ends of bag in dish.
Place dish in oven, allowing room for bag to expand during cooking without touching heating elements, wall, or racks. Bag should not hang over dish.
Bake 20 minutes.
Cut open top of oven bag carefully. Remember: Always support bag with the dish.
Serve fish with spinach and lemon, and spoon any juices over the top. Sprinkle with crushed red pepper, if desired.
Making perfectly juicy and delicious fish doesn't have to be so difficult. Use an oven bag to keep in the flavor and simplify cleanup. Add vegetables to the bag and have a heart-healthy meal for your family in no time!