In a large skillet, heat vegetable oil over medium-high heat. Add the chicken pieces and cook until browned and cooked through, about 5-7 minutes. In a small bowl, whisk together soy sauce, honey, rice vinegar, sesame oil, garlic, ginger, and red pepper flakes if using. Pour this sauce over the cooked chicken in the skillet. Stir well to coat the chicken evenly. Let the sauce simmer for about 3-4 minutes, allowing it to thicken slightly. Sprinkle toasted sesame seeds over the chicken and toss to combine. Remove from heat and garnish with sliced green onions. Serve hot over rice or noodles for a delicious meal!