Chop the onion into fine cubes and place in a pan with the bacon. Fry for about 5 minutes and lightly brown in the process. Then deglaze with the balsamic vinegar.
Mix the sunflower oil with the white vinegar, mustard, salt and pepper in a mixing bowl. Stir in the bacon with the onions and let the sauce steep a little.
In the meantime, wash the lettuce and spin dry. Cut the leaves into thin strips and place in a large enough bowl. Pour the sauce over the salad and mix everything well.