Add the water and cornstarch to a jar or a bowl; stir using a fork or small whisk.
Add in the soy sauce, hoisin sauce, brown sugar, garlic, and ginger. Stir vigorously with a fork or whisk. Stir the sauce again right before adding it to the skillet.
Cook the broccoli to package directions, but use only half the time. You basically just want to defrost it.
Drain all of water from the broccoli before using.
Heat the canola oil in a large skillet or wok over high heat. When the oil is very hot, add the beef. Cook the beef for 2-3 minutes. Stir a few times to brown on all sides.
Lower the heat to medium, add the mushrooms and cook 2-3 minutes, stirring frequently.
Add the broccoli and the prepared sauce to the skillet; stir to combine and coat everything with the sauce.
When the sauce is simmering, lower the heat to medium-low and cook for 4-5 minutes, stirring occasionally.
Serve immediately over white rice or noodles.