Prep Chicken: Toss chicken pieces in cornstarch until well coated.
Brown (Optional but Recommended): Heat oil in a skillet over medium-high heat. Brown the chicken on all sides (about 2-3 minutes). This step enhances flavor but can be skipped if in a rush.
Load the Crockpot: Place chicken into the slow cooker. In a bowl, mix soy sauce, rice vinegar, ketchup, hoisin sauce, brown sugar, garlic, ginger, and red pepper flakes. Pour the sauce over the chicken.
Cook: Cover and cook on low for 3-4 hours or high for 1.5–2.5 hours, until the chicken is cooked through and tender.
Add Cashews: Stir in cashews during the last 15–20 minutes of cooking to keep them crunchy.
Serve: Garnish with chopped green onions and serve over rice or steamed veggies.