Brown the sausage (recommended): Heat a skillet over medium-high and add 1 tablespoon olive oil. Sear the sausage links 2–3 minutes per side until nicely browned.
This step adds deep flavor and helps the sausage hold its shape. If you’re short on time, you can skip this and add the sausages straight to the slow cooker.
Layer the vegetables: Add sliced peppers and onions to the crockpot. Drizzle with the remaining olive oil and sprinkle with salt and pepper.
Scatter in the minced garlic.
Add tomatoes and seasonings: Pour in the diced tomatoes and marinara or crushed tomatoes, if using. Add oregano, basil, and red pepper flakes. Stir the vegetables and sauce together to distribute the seasonings.
Nestle in the sausages: Place the browned sausage links on top of the pepper and onion mixture.
Spoon a little sauce over them.
Cook low and slow: Cover and cook on Low for 5–6 hours or on High for 2.5–3.5 hours, until peppers are tender and sausages are cooked through.
Adjust seasoning: Taste the sauce and add more salt, pepper, or red pepper flakes as needed. If you prefer a thicker sauce, remove the lid and cook on High for 20–30 minutes to reduce slightly.
Serve: Slice the sausages if desired. Spoon into bowls with plenty of peppers and sauce.
Garnish with chopped parsley or basil. Serve as is, over cauliflower rice, or tucked into crisp lettuce wraps.