Wash the tomatoes and basil, peel the onion. Cut everything finely. Mix with oil and 150 g ketchup and season to taste
Remove the dough from the tins, unroll and separate the dough triangles at the seams. Spread 6 with the rest of ketchup and 6 with mustard. Sprinkle with 4 tbsp. roasted onions, leaving a rim all around. Halve the sausages, place 1/2 each on the wide end. Roll up towards the tip. Put 6 croissants on a baking tray covered with baking paper
Whisk the egg and spread it on the croissants. Sprinkle with sesame, cheese and the rest of the fried onions. Bake one after the other in a hot oven (200 °C/circulating air: 175 °C/gas: level 3) for 10-12 minutes. Add the sauce