Marinade the chicken breasts in 1/2 cup of italian dressing for at least 30 minutes, I have did it as long as a day before cooking.
Take the casing off the sausage link, put the sausage in a frying pan and brown the sausage. Then add the chopped pepperoni and chopped onion and cook for about 5 minutes longer. Drain the grease, and mix all of the cheese with the rest of the stuffing.
Take the chicken breasts out of the marinade and cut a slight incision in the thickest part of the chicken breast.
Take a regular spoon and start stuffing the chicken breasts. Be sure not to overfill them. (usually takes about 3 or 4 spoon fulls) Then take a tooth pick to seal the chicken breast.
Place the chicken breasts into a baking dish and then pour the remainder 1/2 cup of italian dressing over the chicken breasts, place some aluminum foil over the baking dish and put in the oven for approximately 1 hour.
Take the chicken out of the oven, and take the aluminum foil off, baste the chicken in the italian dressing left in the pan and continue to bake them for 15 minutes until golden brown.