Peel, quarter and wash the potatoes. Clean spinach and wash thoroughly. Peel and finely chop the onion. Cut the bacon into small cubes
Add 2 tablespoons of oil to the bacon fat and heat. Fry the potatoes in it at medium heat for about 20 minutes, turning from time to time. Heat 1 tablespoon of oil in a saucepan. Sauté diced onions in it, add spinach and let it collapse.
Season with salt and pepper. Bring milk and cream to the boil, stir in the sauce thickener. Add mustard. Season to taste with salt, pepper and sugar. Wash fish and sprinkle with lemon juice. Dab fish dry, season with salt and pepper.
Heat 2 tablespoons of oil in a frying pan. Fry the fish for 5 minutes all around. Add fish, bacon and spinach to the potatoes. Pour sauce over it and heat everything up again briefly.