Heat marinara sauce on the stovetop on low to medium heat.
Slice off bottom of onion halves so they will sit flat on the grill. Brush pepper, broccoli rabe, and onion with olive oil.
Place sausage, broccoli rabe, pepper, and onion on grill. Turn sausages until slightly brown and juices run clear. Cook broccoli rabe until tender and the pepper and onions slightly charred. Remove sausages and vegetables from grill.
Place peppers in a paper bag, close, and cool slightly. Remove and discard the charred skin from peppers, slice into strips. Slice onion halves.
In a bowl add salt, pepper, and olive oil. Toss the peppers, onions, and broccoli rabe to evenly coat.
Place mozzarella in the sandwich roll with sausages, peppers, onion, broccoli rabe, and marinara.
A TIP FROM OUR CHEF:
For sausage sandwiches, add the toppings the moment before you eat. This will keep your rolls crisp all the way through to the last bite.