Heat the vegetable oil in a large pot over medium heat. Add the chopped onion and sauté until golden brown, about 5-7 minutes.
Stir in the minced garlic and ginger, cooking for an additional 1-2 minutes until fragrant.
Add the chicken pieces to the pot and cook until browned on all sides, about 5-6 minutes.
Sprinkle in the turmeric, cumin, coriander, garam masala, and red chili powder. Stir well to coat the chicken and cook for another 2 minutes.
Add the rinsed red lentils, chicken broth, and chopped tomato to the pot. Bring to a boil, then reduce the heat to low and simmer for 25-30 minutes, or until the lentils are soft and the chicken is cooked through.
Stir in the chopped spinach and cook for an additional 5 minutes. Season with salt and lemon juice to taste.
Serve hot, garnished with fresh cilantro.