Soak the porcini mushrooms in 50 ml of hot water. Brush and wash black salsifies carefully, peel and cut them into pieces and immediately put them into water mixed with plenty of vinegar and flour.
(Work best with gloves on.) Drain black salsify, cut into pieces and cook in 1/2 litre of boiling salted water for about 15 minutes. Spread the Rösti on a baking tray and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes.
Heat the oil in a frying pan and press the sausage meat as small dumplings from the skin directly into the pan. Fry all around for about 5 minutes. Add peas to the black salsify 3 minutes before the end of the cooking time.
Drain vegetables and porcini, keep liquid. Finely chop the porcini mushrooms. Heat the fat in a pot and sauté the flour in it. Deglaze with vegetable and mushroom water, wine and cream while stirring, let simmer for about 5 minutes.
Add the mushrooms, black salsify and peas to the sauce and reheat. Season to taste with lemon juice, salt and pepper. Wash the chives and cut into small rolls. Arrange black salsify ragout with Rösti on plates, sprinkle with chives.