Take the fish fillet out of the foil and let it thaw for about 30 minutes. Peel, wash and halve the potatoes and cook them in a pot with salted water for 20-25 minutes. Drain and mash the potatoes and stir in 175 ml milk.
Season to taste with salt and nutmeg. Peel shallots and cut into fine cubes. Melt the fat in a saucepan and sauté the diced shallots over medium heat. Add mustard and fish stock.
Bring to the boil once, add 175 ml milk and cream and season with salt and pepper. Simmer for about 5 minutes. Clean, wash and halve the radishes. Clean, wash and blanch in boiling salted water for about 3 minutes.
Wash the dill, shake dry and pluck the banner. Chop coarsely except for something to garnish. Cut fish fillet into large cubes, add to the sauce and cook for about 7 minutes. Add the radishes and mangetouts and season to taste with salt and lemon juice.
Stir in chopped dill. Pour the mashed potatoes onto the plates and press a depression in the middle. Add the ragout and garnish with the remaining dill.