Heat a skillet over medium heat and add a drizzle of olive oil. Once the oil shimmers, add the uncooked Italian sausages and sear for about 2-3 minutes, turning occasionally until they develop a light golden-brown color on the outside.

Transfer the seared sausages to your slow cooker. Layer sliced bell peppers and onions on top. Sprinkle garlic salt, black pepper, and Italian seasoning over the veggies. Pour marinara sauce over everything.

Cover the slow cooker with its lid and cook on high for 3-4 hours or on low for 6-7 hours, checking for doneness.

Optional: Transfer to a baking pan after cooking. Sprinkle shredded mozzarella cheese over the top and broil for 1-2 minutes.

Serve the sausages alongside the peppers and onions, optionally on toasted low-carb buns.