Tender beef and crisp bell peppers and onions are stir-fried in a rich, savory black pepper sauce. This easy black pepper beef is ready in just 30 minutes, with a simple velveting marinade that keeps the beef tender and juicy.
Optional: Partially freeze the beef uncovered for 45-60 minutes until the exterior is firm for easier thin slicing.
Using a sharp knife, slice the flank steak against the grain at a 45° angle into ¼-inch-thick strips.
Transfer the beef strips to a large bowl. Add the beef marinade ingredients and mix well. Marinate for 15-20 minutes and set aside.
In a small bowl, combine the sauce ingredients until the sugar has fully dissolved and set aside.
Heat the vegetable oil in a large pan or wok over medium-high heat. Once hot, add the beef in a single layer, cooking in batches if needed to avoid overcrowding. Sear on both sides until cooked through with lightly browned edges, about 2-3 minutes. Remove and set aside.
Stir-fry the onions and bell peppers for about 30 seconds until crisp-tender.
Re-whisk the sauce and pour it into the pan. Simmer for about 1 minute until glossy and thick enough to coat the back of a spoon. Return the beef and toss briefly until evenly coated and heated through. Serve immediately.
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