Spray the inside of a 6-quart slow cooker with nonstick cooking spray.
Arrange the sliced onions and green bell peppers evenly across the bottom of the slow cooker.
Place the Italian sausage links in a single layer over the onions and peppers.
Spread the marinara sauce evenly over the sausage and vegetables, using a spoon to cover everything.
Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours, until the vegetables are tender and the sausage reaches a safe internal temperature of 160°F (71°C) for pork or beef sausage.
Remove the lid and spoon the sauce over the sausage and onions before serving.