Heat a skillet over medium heat and add the olive oil. Place the sausages in the pan and brown for 2 to 3 minutes on each side to build flavor.
Transfer the browned sausages to the bottom of the slow cooker.
Add the sliced peppers and onions on top of the sausages. Sprinkle evenly with garlic salt, black pepper, and Italian seasoning.
Pour the marinara sauce over the top, spreading it out to cover the vegetables and sausage.
Cover and cook on High for 3 to 4 hours or Low for 6 to 7 hours, until the sausages are cooked through and the peppers are tender.
If using cheese, sprinkle mozzarella over the top just before serving. For a melted finish, place under the broiler for 1 to 2 minutes until bubbly.
Serve warm as is or in toasted buns with extra sauce and cheese if desired.