Remove sausage from the casing by slicing the sausage casing with a paring knife down the middle. Peel back the casing, and add sausage to a large pan heated up over medium-high heat. Cook sausage, breaking it up with a spatula into smaller pieces as it cooks, for about 7 minutes.
Once the sausage is done cooking, add bell pepper and onions to the skillet. Cook for an additional 5 minutes until the onions and peppers have softened, and turn off the heat.
Assemble the sandwich by spreading mayonnaise (if using) on the bread. Place provolone cheese on one side and top with the warm veggie and sausage mixture. Close the sandwich & serve immediately.