Begin by heating a skillet over medium-low heat. Add cumin seeds and Sichuan peppercorns, toasting for 2-3 minutes until fragrant. Transfer to a mortar and pestle to grind coarsely.

In a large pot, bring water to a boil. Add wide rice noodles and cook according to package instructions, typically 5-7 minutes until al dente. Reserve 1 cup of starchy pasta water, drain, and set aside.

Heat vegetable oil in a large skillet over medium heat. Add minced garlic and ginger; sauté for about 1 minute until fragrant and golden.

Increase heat to high, add lamb shoulder, and stir-fry for 4-5 minutes until browned on all sides.

Lower heat to medium, stir in ground spices, sliced yellow onion, and optionally finely chopped serrano pepper. Add soy sauce, chili oil, sesame oil, rice wine vinegar, and sugar. Cook for 5 minutes, adding reserved pasta water if too thick.

Add cooked noodles to the skillet and toss to coat evenly with sauce. Use more reserved pasta water as needed, and stir for about 2 minutes until warmed through.

Spoon the noodles into bowls and garnish with minced cilantro and sesame seeds. Drizzle with additional chili oil if desired.