Crush black peppercorns and sichuan peppercorns with mortar and pestle. (Crush black peppercorns first, as they are harder and need a heavier hand. Then, empty the bowl and crush the Sichuan peppercorns next.) Or, put them in a plastic baggie and bash them with a rolling pin. The texture should be somewhat coarse and not too finely ground.
Cut chicken thighs into bite-sized cubes, about 1 1/2 inches long. Add crushed peppercorns, corn starch, and salt. Mix thoroughly.
Heat wok on high heat. When the wok starts to smoke, add 1 Tbsp neutral oil and swirl around. Add chicken and sizzle until almost fully cooked (but not quite), about 4-5 minutes. Feel free to scrape down the sides, shake the wok, and toss chicken pieces around with a metal spatula from time to time. The goal is to get nicely charred pieces of chicken with crispy edges.
When the chicken is almost done, add leeks. Mix with metal spatula until the leeks are soft with slightly browned edges, about 2 minutes.
Serve immediately with rice and vegetable of choice. Enjoy!