In a medium bowl, toss the sliced chicken with the baking soda and 1/4 cup of water. Let it sit for 15 minutes. Rinse the chicken thoroughly under cold water, drain, and pat very dry with paper towels. Return to the bowl and toss with 2 tablespoons of cornstarch until coated.
While the chicken rests, prepare the sauce. In a small bowl, whisk together the soy sauce, oyster sauce, Shaoxing wine, sesame oil, white pepper, and the remaining 1 tablespoon of cornstarch until smooth. Set aside.
Bring a small pot of water to a boil. Add the broccoli florets and cook for 1-2 minutes, until crisp-tender and bright green. Drain well and set aside.
Heat the vegetable oil in a large skillet or wok over high heat. Add the garlic and sliced onion and stir-fry for about 20 seconds until fragrant.
Add the coated chicken to the skillet in a single layer. Let it cook for about 1 minute without moving, then stir-fry for another 2-3 minutes until golden and cooked through. The internal temperature should reach 165°F (74°C).
Add the drained broccoli back to the skillet. Give the sauce a quick stir and pour it into the pan along with 1 cup of water.
Stir everything together and bring to a simmer. Cook for 1-2 minutes, stirring constantly, until the sauce thickens and coats the chicken and broccoli.
Serve immediately over cooked rice.