Onions peel, halve and in fine strips cut. Wash and clean the tomatoes thoroughly and cut them into halves or quarters. Heat oil in a pan, sauté onions for about 1 minute, dust with sugar and deglaze with vinegar.
Mix tomatoes with it, season with salt and pepper and marinate for about 30 minutes.
Wash basil, shake dry, pluck leaves from the stalks and chop finely. Grate cheese finely. Mix panko, basil, egg yolks, cheese and olive oil, season with salt and pepper. Pour the tomatoes into a sieve and put them into an ovenproof dish.
Spread the Panko cheese mixture on top and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 8-10 minutes. Remove from the oven and arrange.