Make Sauce: In a medium bowl, whisk cornstarch and rice vinegar until smooth. Add the rest of the sauce ingredients, stirring to combine, and set aside.
Cube chicken into 1-inch pieces and thinly slice the bell pepper.
Heat toasted sesame oil in a large nonstick pan until shimmering.
Add the sliced bell pepper and sauté for 3-5 minutes until crisp-tender.
Add cubed chicken and cashews, scraping in the sauce. Cook gently for 1-2 minutes until warmed through.
Remove from heat and drizzle remaining toasted sesame oil over the dish.
Serve over rice and garnish with green onions and sesame seeds.