Place the Italian sausage links, green, red, and yellow bell peppers, onion, garlic, salt, and black pepper into the slow cooker. Add the Italian herb blend and crushed red pepper flakes if you like a touch of heat. Finally, pour the marinara sauce over the mixture. Stir everything together until the ingredients are well combined.
Cover the slow cooker with its lid and cook the sausage and vegetable mixture on High heat for 3-4 hours or on Low heat for 6-8 hours. The sausages should be fully cooked and the vegetables tender by the end of the cooking time. I like to set it on Low and let the flavors meld together all day for maximum taste.
Carefully remove the sausages from the slow cooker and slice them into thick pieces. Return the sliced sausage to the slow cooker and gently mix everything to ensure the flavor is distributed throughout.
Transfer the sausage and pepper mixture to a serving dish. Sprinkle the chopped fresh parsley on top as a flavorful garnish just before serving. For even more freshness, I sometimes add a squeeze of lemon juice right before serving.