Cut the beef in half lengthwise (with the grain), then cut each half on the diagonal (against the grain) into 1/2-inch-thick strips, about 3 inches long.
Whisk together the soy sauce, rice wine, sesame oil, cornstarch, and 1/4 cup water in a small bowl; set aside.
Put 1 tablespoon of the oil in a deep skillet over high heat. Add the garlic, onion, and ginger, and stir-fry until the onion is clear.
Add the beef and stir-fry for 3-4 minutes.
Remove the beef and season with salt to taste.
Add the remaining I tablespoon of oil to the skillet and add the red pepper and bok choy. Stir-fry for 2 minutes, and return the beef to the wok.
Slowly stir in the soy sauce mixture. Cook for 1 minute, or until the sauce thickens slightly. Season with salt as needed.
Serve over the rice.