Clean the fish fillet and sour with lemon juice. Clean and dice the turnip, carrots and potatoes.
Heat the oil in a wide saucepan and first sweat the diced turnip in it. Add the potato and carrot cubes and let them sweat a little. Deglaze with vegetable stock and bring to the boil with a little salt and the bay leaves. Simmer over a mild heat for about 20 minutes until the cubes are cooked through.
Pour the stock into a small saucepan and add the cream. Bring to the boil and reduce a little. Work in cold flakes of butter and season with salt and pepper. Either add the sauce back to the ragout or serve separately.
In the meantime, pat the fish fillets dry, season with salt and pepper and fry them in a pan with a little oil.