Drizzle a small amount of grape-seed or vegetable oil in a large skillet over medium/medium-high heat. Add half or a third of the sausages, then brown for a minute or two on all four sides, or until golden brown, then transfer to a 6-quart crock pot. (Sausage does not need to be cooked all the way through. Repeat with remaining sausage then add to crock pot.
Turn heat up to medium-high then add onions to skillet, season with salt and pepper, and saute until just beginning to become tender, 3-4 minutes then add to crock pot.
Add bell peppers to skillet, season with salt and pepper, then saute until crisp tender, 4-5 minutes, adding a drizzle more of oil if necessary to keep peppers from burning. Add garlic then saute for 1 more minute, and then add peppers and garlic to crock pot
Add red chili pepper flakes, a little bit of salt and pepper, tomato sauce, crushed tomatoes, and chicken broth to crock pot then cover and cook on low for 5-6 hours or until sausages are very tender. Serve over pasta, rice, or inside rolls with provolone cheese on top.