In a saucepan, blanch the sausages in boiling water for 5 minutes. Drain and reserve.
In a skillet, sauté the onion in a little oil until caramelized. Add the first amount of vinegar and cook over medium heat until evaporated. Set aside.
In a skillet, cook the sausages in a little oil, just to brown. Set aside.
In a bowl, toss the arugula with a little oil, the second amount of vinegar, the salt and pepper. Place an equal amount on the bottom half of each roll and arrange the tomato and Bocconcini cheese slices on top.
Cut the sausages in half lengthwise and place on top of the cheese.
Garnish with caramelized onions and close the sandwich
Tip: Sweet potato chips and a salad of bitter greens are the perfect accompaniments.