Preheat the oven to 300°F. Place the cashews on a baking sheet in a single layer. Toast in the oven until fragrant, stirring or shaking pan once, about 15 minutes.
Trim any fat from chicken and cut into bite sized pieces.
Slice green onions in half (white vs green) and separate. Cut into one inch pieces, but keep the whites separate from the greens.
Heat half of the oil in a wok or large saute pan over high heat. When oil is almost at its smoke point, add half of the chicken. Stir only to prevent burning and cook until golden brown. Transfer to a plate. Add remaining oil and once oil is hot, add remaining chicken along with the white sections of the green onion. Allow to brown while occasionally stirring. During last minute or so, add the garlic. Once this half of the chicken has browned, add first batch back to the wok and reduce heat to medium.
Add rice vinegar, stir, and cook until evaporated. Add hoisin, soy, and fish sauces. Stir well and allow to cook until all chicken pieces are fully cooked through. This shouldn't take more than a minute.
Turn off heat and stir in roasted cashew pieces, remaining green onions, and toasted sesame oil.