Slice each sausage link lengthwise, remove the casing and cut into ½ inch, bite-size pieces.

Heat a large skillet on the stovetop over medium high heat. Add the sausage and sear the outside until starting to brown and caramelize. Transfer the sausage to a plate and set aside.

Add the olive oil and onion to the hot skillet. Cook the onion over medium high heat until turning transparent. Add the garlic, red and black pepper, and continue cooking for one more minute or until the garlic is fragrant.

Transfer the sausage and onion & garlic mixture to the crockpot. Add salt, lentils, celery, carrots, chicken stock, canned tomatoes, bay leaf, fennel seeds, and rosemary. Cover and turn crockpot to high for 4 to 6 hours, or 8-10 hours on low. Add more chicken stock if the soup gets too thick. Stir occasionally.

Serve soup with crusty bread and shaved parmesan cheese.