In a medium bowl, season the chicken with salt and pepper then toss with the cornstarch until the chicken is coated.
In a large nonstick pan, heat the oil over medium-high heat. Once shimmering, add the chicken and leave undisturbed for 1 minute to develop a crust. Then sauté, tossing often, until browned and cooked, about 8 minutes.
Add the onion, cook for 2 to 3 minutes until soft and translucent, stirring frequently.
Add garlic and cook for 15 seconds until fragrant.
Add the rice vinegar and deglaze the pan, scraping up any brown bits, about 1 minute.
Reduce heat to medium-low. Add the hoisin, water, and chili garlic sauce. Cook, tossing to combine and warm through, about 1 more minute.
Remove from heat and stir in the scallions and cashews. Taste, season with a little more salt and pepper, if necessary.
Serve over cooked, hot white rice and enjoy!