Place the frozen Rösti on a baking tray. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 20 minutes. Turn once after 12 minutes. In the meantime, peel the onions and cut into fine slices. Clean, wash, quarter and seed the tomatoes. Cut the flesh into small cubes.
Cut the bacon into pieces and let it crisp in a pan. Add olive oil. Steam the tomatoes for about 2 minutes. Add onion slices. Season with salt, pepper, sugar and vinegar. Arrange on the Rösti and serve with tender sage leaves