Cook the aromatics. Heat the olive oil in a large saucepan or Dutch oven over medium-high heat. Add the onion, celery, and carrots. Cook for about 6-8 minutes or until the onion is translucent and yellow, stirring frequently. Add the garlic and cook for about 1 minute longer.

Simmer. Add the tomatoes, tomato sauce, chicken stock, lentils, curry powder, cumin, coriander, and pepper. Bring to a boil. Reduce heat to medium, cover the pan, and simmer for 30 minutes.

Finish with the chicken and coconut milk. Add the diced chicken. Cover and cook for about 15 minutes, or until the lentils are tender. Stir in the coconut milk, brown sugar, and cilantro. Heat through (do not boil) and serve in bowls over hot cooked rice.