Peel and finely chop the onion. Mix vinegar, diced onion, mustard, salt and pepper. Whip the oil into it. Drain the mozzarella balls and marinate in the vinaigrette for about 10 minutes.
In the meantime, sort, clean, wash and drain the spinach. Clean and wash the radicchio and cut into fine strips. Mix the spinach, radicchio, salami, mozzarella and vinaigrette in a large bowl. Arrange on a plate and garnish with tomato slices