Wash and peel the potatoes and cook them in boiling salted water for about 20 minutes. Peel carrots, cut them diagonally into slices. Rinse fish fillet, dab dry, cut into 8 pieces. Season with salt and pepper.
Cook carrots in 2 tablespoons of hot oil for 6-8 minutes until firm to the bite, stirring frequently. Remove the carrots and keep warm. Heat 1 tablespoon of oil in a frying pan and fry the fish fillets for about 5 minutes on each side.
For the sauce, melt 30 g of fat, sauté the flour in it while stirring. Add stock and 100 ml milk while stirring continuously, bring to the boil. Stir in mustard. Season with salt and pepper. Drain the potatoes.
Heat 200 ml milk and 20 g fat, add to the potatoes and mash everything with the potato masher. Season with nutmeg, salt, lime juice and zest. Wash tarragon, dab dry. Put some leaves aside for garnishing.
Chop the rest. Stir half into the sauce. Arrange the pike-perch with carrots, mashed potatoes and sauce. Sprinkle the rest of the chopped tarragon on top, garnish with tarragon leaves and lime slices as desired.