Toast Spices: In a dry skillet over medium heat, toast the cumin seeds and peppercorns for 1–2 minutes until fragrant. Use a mortar and pestle to crush them lightly.
Season the Lamb: In a large bowl, mix the crushed spices, ground cumin, salt, and dried chiles. Toss the lamb strips in this mixture to coat thoroughly.
Prepare the Skillet: Heat a large skillet or wok over high heat until very hot, about 5 minutes. Add peanut oil and let it heat until shimmering.
Cook Vegetables: Add onion slices and scallion pieces to the skillet, stirring until lightly charred but crisp, about 2 minutes. Transfer to a bowl.
Cook Lamb: Add lamb strips and chiles to the skillet. Stir-fry until the meat begins to brown and smoke lightly, about 2 minutes. Add garlic, soy sauce, and sherry. Continue stirring until the liquid has mostly evaporated and the lamb is fully cooked, about 2 minutes.
Combine and Garnish: Toss the onions and scallions back into the skillet and stir well. Remove from heat, mix in cilantro, and garnish with sliced scallion greens. Serve immediately.