Heat peanut oil in a wok or large, nonstick skillet over high heat. Stir in garlic, ginger, orange zest, and red pepper flakes, and cook until garlic begins to brown, 20 to 30 seconds. Add beef; cook and stir until beef begins to brown and crisp, about 5 minutes. Whisk together 1 tablespoon soy sauce, orange juice concentrate, rice vinegar, 1 teaspoon sesame oil, brown sugar, and 1 teaspoon cornstarch in a small bowl. Stir into beef, and cook until sauce has thickened and turned clear, about 30 seconds.
