Prep the Ingredients: Sort 12 oz lentils on a sheet pan, removing debris, then rinse and drain. Season 1 lb chicken thighs with 1 tsp salt. Dice 1 onion, 1 celery rib, and 1 carrot; mince 2-3 garlic cloves. Wash 5-6 oz greens if needed.
Brown the Chicken: Heat 1 tbsp olive oil in a large pot over medium heat. Add chicken thighs and cook 4-5 minutes per side until browned. Remove chicken to a plate, cover loosely with foil, and set aside (it will finish cooking later).
Cook the Vegetables: In the same pot, add diced onion, carrot, and celery. Cook over medium heat for 3 minutes, stirring frequently. Add garlic, 1 tbsp tomato paste, 1 tsp oregano, and ¼ tsp black pepper; cook 2 minutes, stirring constantly.
Simmer the Soup: Add 8 cups chicken broth, sorted lentils, and reserved chicken (with juices). Stir well, bring to a boil over high heat (5-6 minutes), then reduce to medium, cover, and simmer for 20-25 minutes, stirring occasionally, until lentils are nearly tender.
Shred Chicken and Add Greens: Remove chicken from the pot and shred using two forks. Add 5-6 oz greens to the soup, stirring until wilted. Return shredded chicken to the pot and cook 2-3 minutes until heated through and lentils reach desired tenderness. Stir often to prevent sticking.
Adjust and Serve: Taste and add salt if needed (broth and paste may be salty enough). Serve hot alone, with crusty bread, or over white rice for a complete Chicken Lentil Soup meal.