Remove sausage from casings and saute in a large, heavy bottomed pot until nicely browned (but not necessarily cooked all the way through). Remove and set aside. Drain pot of all but about 1tbsp of fat.
Saute diced onion until translucent, scraping up any brown bits from the sausage. Add diced celery and carrots, stirring occasionally but allowing everything to get lightly browned. Add smashed garlic cloves.
Add chicken broth (and bay leaf if using) and bring to a boil
Add the mix of green and yellow lentils (make sure to rinse and pick through for stones first). Bring back up to a boil, then reduce to a lower simmer until lentils are cooked through and soft (around an hour, sometimes more).
Once lentils are cooked all the way through, remove the bay leaf and blend the soup using an immersion blender (or in batches in a blender, being careful with boiling liquid); blend to the desired texture.
Serve immediately with garlic bread and a simple green salad. Soup will keep in the fridge for ~1wk (and makes awesome lunch offers); alternatively it can be stored in the freezer almost indefinitely.