Method:

1. Preheat oven to 190°C. 

2. Rinse fish fillets under cold water, rub with salt and pepper, and place in a shallow dish. Sprinkle lemon juice over fillets and set aside for 15 minutes. 

3. Put flour in a shallow baking dish and roll fillets in flour to coat lightly. In a frying pan, melt 3 tbsp. of butter over medium heat. Fry each floured fillet for 3 to 4 minutes on each side, or until fish turns opaque. Place fish in a baking dish. 

4. Wash and dry frying pan. Melt remaining 2 tbsp. butter over medium heat. Sauté onions for 5 to 10 minutes, or until golden. 

5. Use a knife or rubber spatula to spread mayonnaise or sour cream over fish. Place onions on top and sprinkle cheese over all. 

6. Place dish in oven and bake for 10 to 15 minutes, or until surface is browned and bubbly. Remove from oven, sprinkle with dill, and serve hot.