Sort the lentils and remove any foreign objects, such as tiny rocks, twigs, misshaped beans, etc. Rinse the lentils with cold water and drain.
Season the chicken thighs on both sides with the salt.
Heat the olive oil in a large pot over medium heat. When the oil is hot, but not smoking, add the chicken. Cook for 4-5 minutes. Turn and cook another 4-5 minutes to brown on both sides.
Take the pot off the heat. Remove the chicken from the pot and place them on a pan or plate, keep warm. Don’t worry that the chicken is not cooked through, it will finish cooking in the lentil soup.
Place the pot back over medium heat and add the onions carrots and celery. Gently cook the vegetables for 3 minutes, while stirring frequently.
Add the garlic, tomato paste, oregano and black pepper to the pot. Cook for 2 minutes, stirring almost constantly.
Add the chicken broth, lentils and the reserved chicken thighs (including any juices collected in the pan) back to the pot. Stir well.
Raise the heat to high and bring the broth to a boil.
Lower the heat to medium when the broth is at a full boil. Cover the pot and cook the lentils for approximately 20-25 minutes, stirring occasionally.
Taste the lentils to see if they have almost reached the desired tenderness. If they are still a bit hard for your taste, continue cooking them for 10-15 more minutes and taste again. Stir more often as they thicken.