Wash sage. Wash the fillet and dab dry. Season with salt and pepper. Cover fillet with sage leaves and wrap with bacon. Crush juniper berries. Put fillet and juniper berries in an ovenproof dish.
Heat 150 g butter and pour over the fillet. Roast in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 45-50 minutes. After about 15 minutes roasting time, pour on half of the stock.
Add the rest of the stock little by little. Stir in cream about 15 minutes before the end of the frying time. Meanwhile peel, wash and chop the potatoes. Cook in salted water for about 20 minutes. Then drain. Warm up the milk.
Add milk and 2 tablespoons of butter to the potatoes and mash. Season to taste with salt and nutmeg. Remove the meat. Pass the sauce through a fine sieve into a small pot. If necessary, thicken with sauce thickener.
Season the sauce with salt, pepper, mustard and lemon juice. Arrange everything. Serve with lamb's lettuce.