Lentil Sausage Soup

1 pound lentils (1 bag)
1/4 c. olive oil, plus extra for serving
4 c. diced onion (3 large)
1 tbl. garlic, minced
1 tbl. kosher salt
1 1/2 tsp. freshly ground black pepper
1 tbl. thyme
1 tbl. ground cumin
3 c. medium diced celery (abt. 8 stalks)
3 c. medium diced carrots (abt. 4-6)
1/4 c. tomato paste
1 32 oz. box of chicken broth (you may want to add more if it seems really thick, or just add some water)
2 tbl. red wine vinegar
1 pound kielbasa, cut into chunks or 1 lb. ground Italian sausage
grated Parmesan cheese, for serving

*At times, I've also added 1 14.5 oz. can of diced tomatoes in addition to the tomato paste. If you feel like cleaning them, you can add 4 cups of chopped leeks, white and light green parts only, (abt. 2 leeks), but I never add them.

In a large bowl, cover the lentils with boiling water and allow to sit for 15 minutes. Drain.

Brown and drain the kielbasa/sausage.

In a large stockpot over medium heat, heat the olive oil and saute the onions, leeks if desired, carrots and celery until tender/translucent. Add the garlic, salt, pepper, thyme and cumin. Saute for a few minutes longer. Add the chicken broth, tomato paste (diced tomatoes if wanted) and drained lentils. Bring to a boil, reduce heat and simmer uncovered for 1 hour, or until the lentils are cooked through and tender. Check the seasonings. Add where needed. Add the kielbasa/sausage and vinegar and simmer until they are hot. Serve drizzled with olive oil and sprinkled with grated Parmesan cheese.