Remove lamb leg roast from the refrigerator and let rest at room temperature for 1 hour.
Set oven rack to the middle and preheat oven to 500°F/260°C.
Combine Chinese five-spice powder, ground cumin, salt, garlic powder, onion powder, ground Sichuan peppercorn, and all-purpose flour in a bowl and mix well.
Tip the seasoning over the roast and generously rub into the meat.
Elevate lamb roast onto a roasting rack and place into oven.
Roast at 500°F/260°C for 25 minutes and then turn the oven off. DO NOT OPEN oven door. Let the meat roast for 2 hours undisturbed.
Remove roast from oven, check the internal temperature for doneness, cover loosely with aluminum foil and let rest for 10 minutes.