In a saucepan, fry the onion cubes in melted butter until translucent. Place the fish on the glassy onions. Then sauté gently, bring to the boil with white wine and fish stock. Let it steep for 5-7 minutes. Then carefully remove the fish and keep it warm.
Reduce the remaining stock with cream by half.
Let the potato balls cook in salted water for a good 10 minutes.
Finally, season the sauce with salt, pepper and lemon juice, serve with potato balls and fish fillet.