In a small bowl, whisk together the soy sauce, cornstarch mixed with 2 tablespoons water, brown sugar, rice vinegar, and 3 tablespoons water until smooth; set the sauce aside.
Heat 1 tablespoon vegetable oil in a large skillet over medium-high heat until hot.
Add the minced garlic and minced ginger to the skillet and sauté for about 30 seconds until fragrant.
Add the broccoli florets, sliced mushrooms, and 1/2 teaspoon onion powder to the skillet; stir-fry for about 5 minutes until the vegetables are crisp-tender.
Pour the prepared sauce over the vegetables, stir to coat, and cook for about 2 minutes until the sauce thickens and glosses the vegetables.
Remove from heat and drizzle with 1 teaspoon sesame oil if using, then toss and serve.