Parboil the sausage over medium heat for about twenty minutes with enough water to barely cover the bottom of a skillet

Increase the heat and brown the sausage lightly, about five minutes

Cut the sausage lengthwise almost but not quite all the way through and spread it out flat

Cut the sub roll in the same manner as the sausage and place the sausage in the center of the roll

Sprinkle the mozzarella cheese evenly over the sausage and place the sub, open side up, in a preheated broiler for two or three minutes or just until the cheese is melted

Remove from the broiler, close the sandwich, and cut into serving pieces while still warm.