Cut the beef lengthwise into thin 2-inch long strips. 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. Divide among plates and serve.