Soak porcini mushrooms in 3/4 litres of water for about 1/2 hour. Peel onions, dice 2 of them. Cut bacon into cubes. Clean mushrooms, rub with kitchen paper and cut into quarters. Season roulades with salt, pepper and thyme.
Mix bacon, diced onion and half of the mushrooms and spread on the roulades. Roll up the roulades and pin them with wooden skewers. Heat oil in a roaster. Put the roulades in it for about 5 minutes all around a
Place the Rösti Taler on the fat pan of the oven. Preheat the oven (electric cooker: 225 °C/ circulating air: 200 °C/ gas: level 4). Place the baking tray in the oven. Turn down the temperature (electric oven: 200 °C/ convection oven: 175 °C/ gas: level 3).
Bake thalers for 20-25 minutes. Clean, wash and cut leek into rings. Cut 2 onions into slices. Heat the fat in a pan. Fry the onion slices and mushrooms for about 4 minutes. Add the leek, onion slices and mushrooms to the roulades approx. 10 minutes before the end of the cooking time.
Add the sauce thickener while stirring and bring to the boil. Season to taste with salt and pepper. Serve the roulades with the Rösti-Talers. Garnish with thyme.