Place the sausage links, sliced bell peppers, sliced onion, marinara sauce, Italian seasoning, and red pepper flakes into the slow cooker.
Stir gently to coat the sausages and vegetables with the sauce.
Cover and cook on low for 6 to 8 hours until the sausages are fully cooked and the vegetables are soft.
Remove the sausages, slice them into rounds, and return them to the slow cooker.
Slice the polenta into rounds and warm the slices in a skillet or microwave.
Spoon the sausage and pepper mixture over the warm polenta slices to serve.