Brown the sausage links on all sides in a large skillet over medium heat. This helps to enhance their flavor and texture. Transfer the browned sausages to the slow cooker.
Add the bell peppers, onion, and garlic to the bottom of a 5-quart or larger slow cooker.
Arrange the browned sausages over the top of the vegetables.
Pour the can of crushed tomatoes and the chicken broth over the sausages and vegetables.
Add the Italian seasoning, dried basil, red pepper flakes (if using), kosher salt, and pepper to the slow cooker. Stir to combine all of the ingredients.
Cover the slow cooker and cook on low heat for 5-7 hours or on high heat for 3-4 hours until the sausages are cooked through and the peppers are tender.
Optional Step: Once cooking is complete, remove the sausages from the slow cooker and slice them into bite-sized pieces. Return the sliced sausages to the slow cooker and stir to combine with the peppers and sauce.
Serve on a hoagie roll or with rice or mashed potatoes.