In a mixing bowl, combine the cubed lamb leg with soy sauce, Shaoxing wine, and salt. Toss to coat. Cover with plastic wrap and marinate for 20 minutes.

In a small bowl, mix cumin powder, Sichuan chili flakes, sugar, and ground Sichuan peppercorns. Set aside.

Drain excess liquid from marinated lamb. Sprinkle cornstarch over lamb cubes and toss gently to coat evenly.

Heat 3 tablespoons of peanut oil in a skillet over medium-high heat. Add lamb in a single layer and sear for about 1 minute on each side until golden brown. Set aside.

In the same skillet, add remaining tablespoon of peanut oil, dried chili peppers, ginger, and garlic. Stir-fry for about 30 seconds until fragrant.

Add diced white onion to the skillet and stir-fry for about 1 minute until slightly softened.

Return seared lamb to the skillet, mix well with vegetables, and sprinkle spice mix over. Stir for about 1-2 minutes and adjust salt to taste.

Stir in chopped cilantro and cook for an additional 30 seconds. Serve immediately, garnished with toasted sesame seeds if desired.