Combine the marinade ingredients in a medium bowl. Add the sliced beef and let stand at room temperature for 15-20 minutes.
Heat one tablespoon of vegetable oil in a wok or large skillet over medium-high heat. Add the beef and cook until meat is just browned on the outside, but still pink in the middle. Remove to a plate and set aside.
Add the garlic and ginger to the wok and cook until fragrant, 1-2 minutes. Add the peppers, onion and black pepper and cook until peppers are just crisp-tender. Season with salt, to taste.
Return the beef to the wok and cook until the beef is cooked through, about 1-2 more minutes. Serve over hot cooked rice with chopped green onions for garnish, if desired.