Place the frozen Rösti on a baking tray lined with baking paper. Bake in a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for about 20 minutes. Turn once after about 12 minutes
Onions peel and cut into very fine slices. Wash, quarter and seed the tomatoes. Finely dice the flesh. Cut bacon into pieces
Fry the bacon in a pan without additional fat until crispy. Drain on kitchen paper. Heat the oil in the bacon fat. Sauté onions until translucent. Add tomatoes and steam for about 1 minute. Season to taste with salt, pepper, sugar and vinegar. Garnish with sage