Place the Italian sausage links in the bottom of the slow cooker. Add the sliced bell peppers, sliced onion, and minced garlic on top.
Pour the marinara sauce over the vegetables and sausage. Sprinkle the Italian seasoning, salt, and black pepper across the top.
Cover and cook on low for 6 hours or on high for 3 hours until the sausage is fully cooked and the peppers are soft.
Remove the sausage links from the slow cooker and slice them into rounds. Return the slices to the sauce and stir gently to combine.
While the sausage rests, spiralize the zucchini into noodles. Do not add the noodles to the slow cooker.
Divide the zucchini noodles among four plates. Spoon the sausage, peppers, and sauce over the noodles.
Finish each plate with a sprinkle of grated Parmesan cheese before serving.