Trim the fat from the beefsteak and cut the beef lengthwise into 2-inch strips. Cut the strips crosswise into ¼-inch slices. In a medium bowl, toss the beef with 2 teaspoons cornstarch, 1 teaspoon vegetable oil, the salt, and pepper. Cover and refrigerate for 30 minutes.

Cut the onion into 1-inch squares. Cut the tomatoes into 8 wedges. Wash the basil and pat dry. In a small bowl, combine the remaining 2 teaspoons cornstarch and 1 tablespoon water and set aside.

Heat a nonstick skillet over high heat. Add the remaining 4 teaspoons vegetable oil, the beef, garlic, and onion and stir-fry for 1 minute, or until the meat browns. Continue stirring for about 2 minutes over high heat. Stir in the hoisin sauce and cornstarch mixture. As the mixture begins to thicken, after about 1 minute, add the tomatoes and basil; stir for 1 minute.