In a small bowl or jar, combine the water and cornstarch. Stir with a fork or small whisk until smooth.
Add the soy sauce, hoisin sauce, brown sugar, garlic, and ginger. Mix well to combine. Stir (or shake) the sauce again right before adding it to the skillet.
Cook the broccoli according to package directions, but only for half the recommended time. You just want it thawed and slightly tender. Drain well and set aside.
Heat the oil in a large, deep skillet, sauté pan, or wok over medium-high heat.
When the oil is hot, add the beef and sprinkle with salt. Cook for 3–5 minutes, stirring often, until the beef is browned and any liquid released has mostly evaporated (cooked out).
Reduce the heat to medium. Add the mushrooms and cook for 2–3 minutes, stirring frequently, until they’ve softened and most of their liquid has cooked out.
Add the broccoli, stir. Pour the prepared sauce to the skillet. Stir to combine, coating all the ingredients.
Once the sauce begins to simmer, lower the heat to medium-low. Cover and cook for 3–5 minutes to let the flavors come together and finish cooking the broccoli. Stir occasionally.
Taste and add salt if needed. As a reference, we did not add extra to ours.
Serve over white rice or noodles, with crushed red pepper for a little kick if desired.