Steam broccoli in a steamer basket over a saucepan filled with an inch of boiling water until bright green, 1 minute. Remove the steamer basket from the saucepan and set aside to cool.

In a medium bowl, whisk together the garlic, ginger, soy sauce, and 2 teaspoons of the sherry or rice wine, the cornstarch, salt, and pepper. Add the beef and toss to coat.

In a small bowl, whisk together the hoisin sauce, Sriracha, and last tablespoon of sherry or rice wine. Set aside.

Heat a 14-inch wok or skillet over high heat, until a drop of water vaporizes within 1-2 seconds. Add a tablespoon of the peanut or sunflower oil, then add the beef in a single layer. Cook, undisturbed, for 1 minute. Then stir-fry for 1 minute, until lightly browned but not cooked through. Transfer the beef to a dish and set aside.

Add the last tablespoon of peanut or sunflower oil, and sauté the onion for 30 seconds, just until translucent. Add the steamed broccoli and stir 15 seconds. Return the beef with its juices to the pan, and toss to combine. Give the reserved hoisin sauce mixture a quick stir, and add it to the pan. Stir-fry until the beef is just cooked through, about 1 minute. 

Serve immediately, and pass more Sriracha sauce at the table, if you'd like.