In a 5-6 quart Dutch oven or heavy bottomed pot, heat two tablespoons of the oil over medium high heat.
Once hot, add sausage meat and break up with a wooden spoon while browning, about 3-4 minutes.
Move the sausage meat to the sides and add the remaining oil along with the leeks, celery and carrots. Stir and cook until the leeks begin to soften, about 2-3 more minutes.
Add in the chopped kale and stir the entire mixture and cook to wilt the kale, about another 2 minutes.
Add in the tomato paste and stir into the mixture. Reduce to medium and cook this for 2 more minutes.
Add the lentils, diced tomatoes and the stock and raise the heat to high.
Bring to a boil, cover and lower to the lowest heat.
Cook covered for 35-45 minutes or until the lentils are soft. Be careful not to overcook the lentils. They should still have a little chew.
Season with the salt and pepper and taste. Season more if needed.
Use a small ladle or soup spoon and remove any fat that floats to the top and discard. (We removed about a half cup of fat.)
This final step is optional but advised. Puree some of the soup but not all by either placing some in a blender or use an immersion blender and puree for a few seconds. This will give the soup a nice creamy texture but still have plenty of body.