Combine the 1 tablespoon ground cumin, 1 tablespoon kosher salt and 1 teaspoon black pepper in a small bowl and sprinkle all of it onto the 2 pounds lamb shoulder pieces, tossing around to coat evenly. Heat heavy pot with a lid over medium high heat until it is very hot. Add the 1 tablespoon vegetable oil and brown the meat on both sides. Work in small batches, being careful not to overcrowd the pot. You want to get a nice dark brown crust on the outside of the meat as this contributes to the flavor of the sauce. Transfer the browned meat to a plate and set aside.
Add a little more oil if the pan needs it, then add the 1 teaspoon whole cumin, ½ teaspoon black mustard seeds, ½ teaspoon fenugreek seeds and ¼ teaspoon fennel seeds to the pot, stirring for about 30 seconds to lightly toast them. Add the 1 tablespoon ginger and 1 tablespoon garlic and fry for another minute or so while stirring continuously to prevent burning. Add the 3 onions, then turn down the heat to medium low and cover with a lid for 10 minutes. Remove the lid, and stir vigorously to loosen the bits of flavor from the bottom of the pan. Let most of the accumulated liquid evaporate, then return the lamb to the pot along with any juices. Stir to combine, then cover with a lid and cook over medium low heat for 1 ½ hours.
Add the 6 - 10 jalapeño peppers and ½ teaspoon tamarind concentrate. Since the capsaicin (the compound that makes chili peppers spicy) is in the light color membranes the seeds are attached to, leaving some of the membranes will make your masala more spicy. partially cover to allow steam to escape, then cook until the lamb is very tender and the sauce is very thick (about 1 to 1.5 more hours).
Serve with rice and some chopped cilantro to garnish.