In a mortar, crush the Sichuan peppercorns until fine but not powdery. Add the salt and lightly crush to combine, leaving it coarse in texture.

In a large saucepan, combine the stock with the star anise, black peppercorns, cinnamon stick, orange zest, and the sliced ginger. Bring to a simmer, cover and cook over moderately low heat until fragrant and flavorful, about 20 minutes. Strain the broth into a clean pot and season with salt and pepper. Add the mushrooms, cover and cook over low heat until tender, about 5 minutes. Keep the broth hot.

In a large saucepan of water, poach the chicken at a very low simmer at low heat just until it is white throughout, about 18 minutes. Transfer the chicken to a cutting board, pat dry and let rest for 5 minutes. Slice the chicken crosswise on the diagonal 1/2 inch thick.

Meanwhile, in a medium bowl, soak the noodles in hot water until pliable, about 5 minutes. Drain and transfer the noodles to the hot broth. Let stand until heated through, about 1 minute.

Ladle the noodles, mushrooms, and broth into shallow bowls. Top with the sliced chicken breasts, scallions, and ginger matchsticks. Sprinkle the chicken with the Sichuan pepper-salt and serve.

Mung bean noodles are also known as cellophane or bean thread noodles. They are available at Asian markets.