Peel, wash and dice the potatoes and celery. Cook in boiling salted water for about 20 minutes. Wash sage, shake dry and pluck leaves. Wash fish, dab dry, cut into 4 pieces and sprinkle with lemon juice.
Season fish pieces with a little salt and pepper. Cover with 2-3 sage leaves and wrap 1 slice of bacon firmly around each. Heat oil in a pan. Fry the fish for 6-8 minutes while turning.
Heat buttermilk and butter slightly. Drain the vegetables and add the buttermilk mixture, then mash everything into a puree. Season to taste with salt and nutmeg. Arrange mashed potatoes with 1 fish saltimbocca each. Drizzle fish frying fat over it and sprinkle with remaining sage leaves.