Peel, wash and chop the potatoes and celery. Cook both in boiling salted water for about 20 minutes. Wash the fish, dab dry and cut into 4 equal pieces. Sprinkle with lemon juice, season with salt and pepper.
Chop peanuts. Mix breadcrumbs and peanuts together. Whisk the eggs, turn the fish fillet lightly in flour, carefully beat off excess flour, then turn in the egg and finally in the peanut crumbs.
Heat the oil in a pan. Spread breaded fish evenly in it and fry on low heat for 4-5 minutes on each side.
Wash the salad and shake dry. Wash and halve the tomatoes. Peel and finely chop the onion. Mix vinegar, 2 tablespoons of cold water and onion, add olive oil and season with sugar, salt and pepper.
Mix lettuce, tomatoes and vinaigrette. Heat milk and butter in a small pot. Drain potatoes and celery, add hot milk-butter mixture and mash with a potato masher.
Season with salt, pepper and nutmeg.
Arrange fish fillet, mashed potatoes and celery and salad on plates. Garnish with lemon wedges and cress. Ketchup tastes good with it.