Combine garlic, cornstarch, 1 teaspoon oil, and black pepper in a large mixing bowl. Add sirloin tips; mix well.

Heat 2 tablespoons oil in a wok over high heat for 1 minute. Add sirloin mixture; cook and stir until beef begins to brown, about 2 minutes. Transfer to a large bowl and set aside.

Heat remaining 1 tablespoon oil in the same wok. Add onion; cook and stir until tender. Mix in green beans, then pour in broth. Cover, reduce heat to medium, and simmer until beans are tender-crisp, 4 to 5 minutes.

Stir in beef and soy sauce. Cook, stirring constantly, until heated through, 1 to 2 minutes.