Melt the bacon fat or butter in the bottom of a large pot over medium heat then add the onion, celery, carrots, garlic, and season generously with salt and pepper. Cook until the vegetables have softened, about 10 minutes. Add the tomato paste and stir to coat the vegetables and cook until the tomato paste darkens, about 2 minutes.

Add the water or chicken stock (remember, the thighs simmering will make the broth) and lentils and bring to a boil. Reduce the heat to maintain a very gentle simmer and add the chicken thighs. Cover and simmer for 1 hour, stirring and checking periodically to make sure the soup is nott bubbling too vigorously and to remove any foam on the surface. Remove the chicken thighs to a plate with a slotted spoon.

Depending how thick or broth like you like the soup, mash about half of the soup until it’s thick and creamy, mash more for cream or less for more broth. Using two forks, remove the skin, bone, and cartilage from each thigh and discard or save as a treat for your dog, and shred the meat and return it to the pot. Taste and adjust seasonings as needed. Let the soup heat, on the low simmer for a few minutes.

Serve with Parmesan cheese sprinkled on the top if desired.