Brown the chicken: In a large skillet, heat olive oil over medium-high heat. Toss the chicken pieces with cornstarch until well coated. Add to the skillet and cook for about 5 minutes, until browned on all sides (the chicken does not need to be fully cooked through).
Make the sauce: In a small bowl, whisk together the soy sauce, hoisin sauce, red wine vinegar, sriracha, sesame oil, chicken broth, sugar, and garlic.
Slow cook: Transfer the browned chicken to your slow cooker. Pour the sauce over the top, stir to coat, and cover with the lid. Cook on LOW for 4 hours or until the chicken is tender and infused with flavor.
Finish & serve: Sprinkle with crushed red pepper flakes (if using) and garnish with fresh green onions or cilantro. Serve hot over white rice, brown rice, or steamed vegetables.