In a medium bowl, season the beef with salt and pepper. Add two tablespoons of cornstarch and mix well.
In another smaller bowl, stir together all the sauce ingredients and set aside.
In a small pan over medium heat, toast Sichuan peppercorns until fragrant; about one to two minutes. Remove from heat and crush half a tablespoon of the peppercorns in a mortar and pestle, or in a plastic bag with a rolling pin. Leave remaining peppercorns whole.
Heat a wok over high heat and when wok starts to smoke, add the peanut oil. Add the remaining whole Sichuan peppercorns, dried chilies, and fresh chilies and stir-fry for a few seconds. Then add the beef and stir-fry for two to three minutes to brown meat. Add the sauce mixture and peanuts and cook until the sauce has reduced slightly and is sticky and has a thicker consistency. Add the scallions and coriander; toss to coat and warm through. Then transfer to a serving plate, garnish with ground Sichuan peppercorns. Serve immediately.