I like to whisk the marinade ingredients in a small bowl and pour it over the sliced chicken. Letting it sit while prepping the other ingredients allows the flavors to penetrate and makes the chicken tender.
Cut the broccoli into bite-sized florets and steam in a vegetable steamer or microwave for 3 minutes. Don’t overcook it because it will continue to cook in the pan with the sauce.
Whisk together soy sauce, brown sugar, minced garlic, flour, rice vinegar, oil, and water in a separate bowl. Setting this aside lets the flavors meld and ensures it thickens evenly later.
Heat a large skillet over medium-high heat with a little oil if needed. Sauté the chicken with its marinade until golden brown, about 5 minutes, stirring frequently.
Add the sauce, steamed broccoli, and optional water chestnuts to the pan. Stir to coat evenly and cook until the sauce thickens, around 2 minutes. For a thinner sauce, gradually add water until desired consistency.