Add the thinly sliced top sirloin to a medium bowl with the soy sauce, cornstarch, rice wine, granulated sugar, & white pepper. Stir to combine, coating the beef thoroughly & evenly. Set aside to rest at room temperature.
Meanwhile, as the beef marinates, prepare the stir fry sauce. Add all listed ingredients – beef stock/broth, oyster sauce, cornstarch, soy sauce, & granulated sugar – to a small bowl. Whisk to combine well, taking care to break up any lumps & bumps of cornstarch. Set aside for stir frying, or store in an airtight container in the refrigerator for up to 5 days.
Add 1 teaspoon of the oil to a large wok or skillet over medium-high heat. Once hot & shimmering, add the carrots & broccoli. Lightly season with a small pinch of salt & cook, stirring often, until vibrantly colored & softened slightly, 3-4 minutes. Transfer to a large plate or bowl & set aside.
Add the remaining 1 tablespoon of oil to the wok or skillet. Add the garlic & give it a quick stir. Once the garlic is fragrant, reduce the heat to medium & carefully add the marinated beef from Step 1. Shake the pan to separate the slices of beef for even browning. Cook, tossing & stirring occasionally, just until browned.
Once browned, push beef to the outside edges of the wok or skillet. Stir prepared sauce, then pour it into the center of the skillet, scraping the bottom of the skillet to release any browned bits from the bottom. Cook until thickened slightly, then add the softened broccoli & carrots. Cook 1-2 minutes longer, tossing & stirring frequently to bring the beef and broccoli stir fry together. Remove from heat.