Toast spices by heating a skillet over medium-low heat. Add cumin seeds and Sichuan peppercorns, toasting for 2-3 minutes until fragrant. Grind coarsely.

Cook wide rice noodles in boiling water according to package instructions for 6-8 minutes. Reserve 1 cup of cooking liquid, drain, and toss with oil.

In a large wok, heat vegetable oil over medium heat, add garlic and ginger, sautéing for 1 minute. Increase heat and add sliced lamb, cooking until browned (3-5 minutes).

To the lamb, add ground spices, sliced yellow onion, and chopped serrano pepper (if using). Pour in the soy sauce, chili oil, sesame oil, vinegar, and sugar. Cook for an additional 5 minutes.

Fold cooked noodles into the lamb mixture, tossing to coat with the sauce. Adjust consistency with reserved cooking liquid and season with salt.

Serve the noodles in bowls, garnished with cilantro, toasted sesame seeds, and extra chili oil if desired.