In a small bowl, whisk together the soy sauce, oyster sauce, sesame oil, cornstarch, and water until smooth to create the stir-fry sauce.
Heat the vegetable oil in a large wok or skillet over high heat until it begins to shimmer.
Add the sliced mushrooms to the wok and sauté for 4 to 5 minutes until they are browned and their liquid has evaporated.
Incorporate the broccoli florets and add one tablespoon of water; cover immediately with a lid to steam for 2 minutes until the broccoli is bright green and tender-crisp.
Remove the lid and add the garlic, ginger, and red pepper flakes; stir-fry for 60 seconds until highly fragrant.
Pour the prepared sauce over the vegetables and toss continuously for 1 to 2 minutes until the sauce thickens and glazes the ingredients.
Remove from heat, garnish with toasted sesame seeds, and serve immediately.