Peel the onion and pour it over the slicer. Slice the washed tomatoes and place in a large bowl with the onions.
Now stir the dressing together from the olive oil, balsamic vinegar, mustard and water and mix in the finely chopped herbs. Now round off the dressing with a splash of sweetener and the lemon and season with salt and pepper. Pour the whole thing over the tomatoes and onions and fold in.
Let the salad stand in the refrigerator for at least an hour.