Peel and wash the potatoes and cook them in salted water for about 20 minutes. Peel, wash and slice carrots.
Heat 1-2 tablespoons of fat. Sweat flour in it. Stir in 1/4 l milk, bring to the boil. Simmer for about 5 minutes. Season sauce with mustard, salt and pepper. Keep warm.
Sauté the carrots in 2 tablespoons of hot fat. Season with salt, pepper and 1 pinch of sugar. Add 5 tablespoons of water and steam covered for 5-7 minutes.
Wash the fish, dab dry and cut into pieces. Salt the fish. Fry in hot oil first on the skin side, then on the meat side for about 3 minutes each.
Drain the potatoes. Add 1 tbsp. fat and 150 ml milk. Mash everything. Season with salt and nutmeg.
Wash the tarragon, cut the leaves finely and stir into the sauce. Serve everything. Garnish with slices of lime.