Peel and wash the potatoes and cook them in boiling salted water for about 25 minutes. In the meantime, wash sage, shake dry and pluck leaves from the stems. Wash fish, dab dry, cut into 4 equal pieces and sprinkle with lemon juice.
Top the fish pieces with 3-4 sage leaves each, wrap 1 slice of bacon firmly around each. Season with salt, except for the bacon, and pepper. Heat oil in a pan. Fry the fish for 5-6 minutes while turning.
In the meantime heat milk and butter. Drain the potatoes and let them evaporate briefly. Add milk and butter and mash everything to a puree. Season to taste with salt and nutmeg. Arrange mashed potatoes and fish sprinkled with frying fat and the rest of the sage on plates.