How to Make It
In a medium skillet, combine the coriander, cumin and mustard seeds with the chiles and peppercorns. Toast the spices over moderate heat, shaking the pan, until fragrant, about 1 minute. Transfer to a spice grinder to cool completely, then grind to a powder. Transfer the spices to a bowl and stir in the turmeric, fenugreek, cardamom and ginger.
Spread the lamb on a large baking sheet and rub it all over with the spices. Cover with plastic wrap and refrigerate for at least 6 hours or overnight.
Preheat the oven to 400°. Scrape most of the spices off the lamb. Rub the meat with 3 tablespoons of the olive oil and season with salt.
In a large roasting pan set over 2 burners, heat the remaining 1 tablespoon of olive oil until shimmering. Add the lamb, fat side down, and cook over moderate heat until deeply browned, about 5 minutes. Turn the lamb over and cook over moderate heat until golden, about 3 minutes. Transfer the lamb to the oven and roast for 25 minutes, or until an instant-read thermometer inserted in the thickest part of the meat registers 120° for medium rare.
Transfer the lamb to a carving board and let stand for 10 minutes. Carve the meat into thick slices and serve.