In a stock pot, heat oil over medium heat. Add chicken cubes and sprinkle with salt and pepper. Saute for 2-3 minutes, stirring occasionally to brown all sides of meat cubes.
Add garlic, ginger, and onion. Saute for another 2-3 minutes. Add dried spices (cumin, mustard, coriander, cinnamon) and brown sugar and stir until combined. Add broth and tomatoes with juice; bring to a boil. Add vegetables. Return to boil, reduce heat and cook for 10 minutes. Add lentils, cover, and simmer for an additional 20 minutes.
Using a fork, check to be sure vegetables are soft. When soft, add spinach, stir to combine until spinach is wilted. Add cilantro.
Serve in soup bowls. Garnish each bowl with a dollop of plain yogurt or sour cream, if desired, and chopped cilantro.