Heat the vegetable oil in a large pot over medium heat. Add the lamb cubes and brown them on all sides for about 5 minutes. Remove the lamb and set it aside.
Add the diced onion to the same pot and cook until it softens, about 4 minutes. Stir in the minced garlic and ginger and cook for 1 more minute.
Add the cumin, coriander, turmeric, and chili powder. Stir for 30 seconds until the spices smell fragrant.
Return the lamb to the pot. Pour in the diced tomatoes with their juice, the chicken broth, salt, and black pepper. Bring the mixture to a boil.
Stir in the rinsed lentils. Reduce the heat to low, cover the pot, and simmer for 45 minutes until the lamb is tender and the lentils are cooked through.
Add the spinach leaves and stir until they wilt, about 2 minutes. Remove the pot from the heat.
Sprinkle the chopped cilantro over the stew before serving.