Soak porcini mushrooms in 450 ml cold water for about 1 hour. In the meantime wash and clean the pumpkin, quarter it and scrape out the seeds. Cut pumpkin into pieces. Wash potatoes thoroughly, cut in half, cut several times lengthwise and crosswise, do not cut through.
Spread the potatoes on about 2/3 of a baking tray, season with salt. Spread butter in flakes on the potatoes and sprinkle with parmesan. Season pumpkin in a bowl with salt and pepper. Drizzle 3 tablespoons of oil over the pumpkin and mix.
Spread on the tray next to the potatoes. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 35-40 minutes.
Peel and finely dice the onion and garlic. Clean, wash and slice the mushrooms. Pour the porcini mushrooms into a sieve lined with kitchen paper and collect the soaking water. Chop the porcini finely.
Dab steaks dry, season with salt. Heat 2 tablespoons of oil in a large frying pan. Fry the steaks for 3-4 minutes on each side. Remove, season with pepper and keep warm.
Sauté the mushrooms in the frying fat. Add the onion, garlic and porcini mushrooms and steam briefly. Deglaze with sherry and reduce over high heat until the sherry is almost overcooked. Dust with flour and sauté briefly.
Stir in soaking water while stirring. Stir in sour cream. Season with salt, pepper and sugar, simmer for about 5 minutes while stirring. Wash the chives, dab dry and cut into fine rolls.
Take the pumpkin and potatoes out of the oven. Arrange rump steaks with sauce, potatoes and pumpkin. Sprinkle with chives.