In a small bowl, mix the cornstarch with the water until smooth and set aside.

Heat the vegetable oil in a large skillet or wok over medium-high heat.

Add the ground beef to the skillet and cook, breaking it up with a spatula, until browned and cooked through, about 5-7 minutes. Season with salt and black pepper to taste, then transfer to a plate and set aside.

In the same skillet, add the sesame oil, minced garlic, and grated ginger. Cook for 30 seconds to 1 minute until fragrant.

Add the broccoli florets to the skillet and stir-fry for 4-5 minutes, or until tender-crisp. If needed, add a splash of water to help steam the broccoli.

Return the cooked ground beef to the skillet and mix with the broccoli.

Stir in the soy sauce, oyster sauce, and red pepper flakes (if using). Pour in the cornstarch mixture and stir well to combine.

Cook for another 2-3 minutes, stirring frequently, until the sauce thickens and evenly coats the beef and broccoli.

Remove from heat and serve hot, optionally over cooked rice or noodles.