Place the Italian sausage links in the bottom of the slow cooker.
Add the sliced red bell peppers and sliced yellow onion on top of the sausages.
Sprinkle the minced garlic, Italian seasoning, salt, and black pepper over the vegetables.
Pour the marinara sauce evenly across the top.
Cover and cook on low for 6 to 7 hours until the sausages are fully cooked and the peppers are soft.
Use tongs to remove the sausages and slice them if desired, then return them to the slow cooker and stir everything together.
Spoon the sausage, pepper, and onion mixture into the hoagie rolls and serve.