Wash the fish, pat dry, sprinkle with 4 tablespoons of lemon juice and leave covered for about 15 minutes. Heat the fat in a saucepan, sweat 30 g flour in it while stirring. Deglaze with stock and cream, bring to the boil while stirring and simmer gently for about 5 minutes.
Season with mustard, remaining lemon juice, salt and pepper and keep warm. Halve the leek sticks lengthwise, wash thoroughly and cut into pieces about 5 cm long. Cook the leeks in boiling salted water at low heat covered for 10-12 minutes.
Sprinkle fish with salt, turn in remaining flour. Heat the oil in a pan and fry the fish fillets for 8-10 minutes at medium heat until golden brown. Drain the leek and let it drain. Arrange the fish and some sauce on the leek.
Sprinkle with coarse pepper. Garnish with cherry tomatoes and parsley. Remaining sauce extra is enough. Parsley potatoes taste good with it.