In a large, heavy bottom soup pot, heat olive oil and butter over medium-high heat. Sprinkle chicken thighs with salt, pepper and 1 tablespoon of the curry powder. Place the chicken in the pot and cook until golden on one side. When chicken no longer sticks to the pan and can be moved, turn over and cook on the other side until golden and just cooked through. Remove to a plate and set aside to cool.
Over medium-high heat, add the onions, carrots, celery and sweet potato to the pot and sauté, stirring until the onion is translucent and the veggies are soft. Add the garlic, apple and ginger and sauté for a couple of minutes. Stir in the lentils, diced tomatoes, remaining curry powder, coriander and turmeric. Add the chicken broth, half the chopped cilantro and stir.
Chop the reserved chicken into bite sized pieces and return to the soup, along with any accumulated juices. Stir, bring soup to a simmer and cook over low heat for approximately 10 minutes. If soup is too thick, add enough water to bring to desired consistency.