Preheat oven to 375°F. Scatter nuts on a small baking sheet and toast 6-8 minutes until fragrant, golden and crisp. Remove and cool; turn off oven.
Season the chicken with salt and pepper.
Heat a large pan, wok or round-bottom pot over high heat. Add oil, 1 turn of the pan. Brown half the chicken and remove to a plate. Add more oil and repeat with remaining chicken. Remove and reserve.
Add remaining tablespoon oil to the pan along with the peppers, leeks, ginger, garlic and chili flakes, and toss to combine. Add vinegar, Hoisin, water, Tamari or Shoyu, and sesame oil, and stir to combine. Add in reserved chicken and cashews, give everything a quick toss to coat, and remove pan from heat.
Spoon rice into shallow bowls, make a well and fill with cashew chicken. Serve topped with sesame seeds and cilantro.