Onions peel, halve and in fine strips cut. Wash the tomatoes. Halve cherry tomatoes, quarter tomatoes. Heat 2 tablespoons of oil in a frying pan and sauté the onions for about 1 minute.
Sprinkle with 2 tbsp. sugar, caramelise and deglaze with vinegar. Stir in tomatoes, season with salt and pepper and fill into an ovenproof dish. Marinate for about 30 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash basil, shake dry, remove leaves and chop finely. Grate cheese finely. Mix with basil, panko, egg yolk and 2 tbsp. oil, season with salt and pepper.
Spread the Panko cheese mixture on the tomatoes. Bake in a hot oven for 8-10 minutes. Arrange crumble. Mashed potatoes go well with it.