Wash the fish fillet and sprinkle with lemon juice. Peel, wash and grate the potatoes. Stir in egg. Season with salt and pepper. Dab the fillets a little bit, turn them in the grated potatoes and press them on.
Heat clarified butter in a frying pan. Fry the fillets for about 12 minutes, turning them over. Clean, wash and cut the leek into pieces. Cook in boiling salted water for 10-12 minutes. Melt butter in a pot.
Add flour and sweat in it. Add stock while stirring and bring to the boil. Stir in cream and mustard. Pluck tarragon leaves from the stalks and chop coarsely. Season sauce with salt and pepper, stir in tarragon.
Drain the leek and add to the sauce. Arrange fish and vegetables on a plate, garnish with tarragon and lemon slices.