Clean leeks. Cut leek in half, length wise. Then chop into 1-inch half moons. Add to large bowl and cover with cold water. Swirl leeks with your fingers, separating the layers. The dirt will loosens and fall to the bottom of the bowl while the leeks remain floating. Transfer leeks into a colander then dump the dirty water. Repeat until there's no more dirt remaining in the bottom of the bowl.
Crush peppercorns. Using a mortar and pestle, plastic baggie and rolling pin, or electric spice grinder. Crush black peppercorns first, as they are harder and need a heavier hand. Then, empty the bowl and crush the Sichuan peppercorns next. The texture should be somewhat coarse and not too finely ground.
Cube chicken. Chop chicken thighs into bite-sized cubes, about 1 1/2 inches long and place in a large bowl. Add crushed peppercorns, corn starch, and salt. Mix thoroughly.
Heat wok on high heat. When the wok starts to smoke slightly, add 1 Tbsp neutral oil and swirl around. Add chicken and spread out in an even layer. Leave the chicken undisturbed for 1-2 minutes, to let a char develop. Then scrape down the sides, shake the wok, and toss chicken pieces around with metal spatula (fish spatula) from time to time. The goal is to get nicely charred pieces of chicken with crispy edges. The total cooking time should be 4-5 minutes total.
When the chicken is 90% cooked, add leeks. Mix with metal spatula until the leeks are soft with slightly browned edges, about 2 minutes more.
Serve immediately with rice and vegetable of choice. Enjoy!