I would suggest you pre-cook the sausage and pat off the grease with a paper towel, then add it to the soup - otherwise you are going to end with with a very greasy lentil soup.
Suggestion - go to your local grocery store and purchase a variety of the smallest pieces of sausage possible. Pan fry, then serve with mustard and mashed potatoes and a green veggie. That way, you will be able to figure out which one tastes better and use that one in your soup.
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