Whisk soy sauce, hoisin, honey, rice vinegar, water, sesame oil, and red pepper flakes in a bowl; set aside.
Pat chicken dry. Toss with cornstarch, salt, and pepper until every piece is lightly coated.
Heat oil in a large skillet over medium-high heat until shimmering.
Sear chicken in a single layer, working in two batches. Cook, turning, until browned and cooked through, 5–6 minutes per batch. Transfer to a plate.
Reduce heat to medium. Add garlic and ginger to the skillet and stir until fragrant, about 30 seconds.
Return chicken to the pan. Pour in the sauce and simmer, stirring, until thick, glossy, and sticky, 2–3 minutes.
Fold in cashews and half the green onions. Garnish with remaining green onions and sesame seeds. Serve hot.