Heat oil in a large pot or dutch oven over medium heat, then add onions and cook until soft and golden, about 15 minutes (this is an important flavour-building step, don't rush it). Push the onions to one side and add the cumin seeds to the other along with another glug of oil. When they're a shade darker and fragrant, add the jalapeno, ginger, and garlic and cook, stirring everything together for a minute. Add remaining spices and stir to coat, then add tomatoes.  Cook this mixture for a few minutes, until you see oil shimmering on the top.

Add salt (about 2 teaspoons of kosher), lamb, water and potato and bring to a boil. Reduce to a simmer, cover and cook until potatoes are tender and lamb is cooked through, about 20 minutes. Taste, and add more salt as needed. Sprinkle with cilantro just before serving.