Heat oil in a large skillet over medium heat. Add lamb, potatoes, onion, cinnamon stick, ginger, garam masala, garlic, jalapeno pepper, coriander, turmeric, salt, and cayenne pepper. Reduce heat and simmer, stirring occasionally, until onion softens, about 20 minutes. Add tomatoes to the skillet with the lamb mixture and stir well. Cover and simmer over low heat until lamb is tender, 30 to 40 minutes more. Garnish with cilantro before serving.