Line a prep cup with foil then set aside.

Heat a large, 12”+ skillet with high sides over medium-high heat. Once hot, spray with nonstick spray then add as many sausage halves as will fit in the bottom. Sear the sausages until golden brown on all four sides (sausages do not need to be fully cooked through) then transfer to a 6-quart crock pot. Pour the fat into the foil-lined prep cup then repeat with the remaining sausages, turning the heat down slightly if necessary to prevent burning.

After the last batch of sausages, pour out all but ~1 Tablespoon fat from the skillet then turn the heat up slightly. Add the peppers and onions to the skillet, season with salt and pepper, then saute until the vegetables begin to soften, 3-5 minutes. Add the garlic and red chili pepper flakes, if using, then saute for one more minute. Scrape the vegetables into the crock pot with the sausages.

Pour the spaghetti/marinara sauce into the crock pot. Add a couple Tablespoons of water into each jar then shake to loosen up any sauce on the sides and pour into the crock pot. Stir to combine all the ingredients then place a lid on top and cook on LOW for 5-6 hours or until the sausages are extremely tender. Use a spoon to scoop off the fat that rises to the top of the crock pot.

Butter and toast rolls or buns under the broiler then place the cooked sausage and peppers plus a little sauce on top. Add a slice or two of cheese then broil until melted. Alternatively you can serve the sausage, peppers and sauce over cooked pasta.