- Cook the ramen noodles according to package instructions (without seasoning packets). Drain and set aside. 
- In a large skillet or wok, heat the vegetable oil over medium-high heat. 
- Add the chicken pieces and season with salt and pepper. Cook until browned and cooked through, about 5-6 minutes. 
- Push the chicken to the side of the pan. Add bell pepper, carrots, and broccoli. Sauté for 3-4 minutes until tender-crisp. 
- Add garlic and cook for another 30 seconds until fragrant. 
- In a bowl, whisk together the soy sauce, oyster sauce, brown sugar, rice vinegar, and sesame oil. 
- Pour the sauce into the skillet and toss to coat the chicken and veggies. If using, stir in the cornstarch slurry to thicken the sauce slightly. 
- Add the cooked ramen noodles and toss everything together until well combined and heated through. 
- Garnish with sesame seeds and green onions before serving, if desired.