Whisk soy sauce, oyster sauce, rice vinegar, sesame oil, ginger, and garlic in a small bowl to create the sauce base.
In a separate small container, mix cornstarch and water to create a slurry.
Heat avocado oil in a large wok or skillet over high heat until shimmering.
Add chicken cubes to the hot pan and sear for 3 to 4 minutes until browned and the internal temperature reaches 165 degrees Fahrenheit. Remove chicken and set aside.
Add broccoli florets and 2 tablespoons of water to the same pan; cover immediately with a lid to steam for 2 minutes until tender-crisp.
Remove the lid, return the chicken to the pan, and pour in the sauce mixture.
Stir in the cornstarch slurry and toss continuously for 1 to 2 minutes until the sauce thickens and coats the chicken and broccoli.
Garnish with red pepper flakes if desired and serve immediately.