Layer the Sausages: Arrange the Italian sausage links in a single layer on the bottom of your slow cooker insert.
Add Aromatics and Vegetables: Scatter the sliced onion, minced garlic, and the sliced red and orange bell peppers over and around the sausages.
Create the Sauce: In a separate medium bowl, whisk together the low-sugar marinara sauce, dry red wine, Italian seasoning, smoked paprika, and optional red chili flakes. Season lightly with salt and pepper, being mindful that the sausage and marinara may already contain salt.
Combine in Slow Cooker: Pour the sauce mixture evenly over the sausages and vegetables in the slow cooker. Give a gentle stir to ensure everything is mostly coated.
Cook Low and Slow (or High and Quicker): Secure the lid on the slow cooker. Cook on the LOW heat setting for 5 to 7 hours, or on the HIGH heat setting for 2.5 to 3.5 hours. The sausages should be cooked through (internal temperature of 165∘F or 74∘C) and the peppers tender.
Slice the Sausages: Carefully remove the cooked sausages from the slow cooker and transfer them to a cutting board. Slice the sausages into 1-inch thick rounds.
Marry the Flavors: Return the sliced sausage to the slow cooker with the peppers and sauce. Stir gently to combine. If possible, allow the dish to rest on the WARM or LOW setting for an additional 10-15 minutes for the flavors to meld further.
Serve and Enjoy: Serve hot. This dish is excellent on its own, or paired with cauliflower rice, zucchini noodles, a fresh green salad, or low-carb bread to soak up the delicious sauce.