Combine soy sauce, honey and 1/4 cup water in a small jug. Set aside.
Toss beef and cornflour in a medium bowl until beef is evenly coated. Heat half of the olive oil in a large wok or large frying pan over medium-high heat. Stir-fry beef, in 3 batches, for 3 minutes or until browned. Transfer beef to a bowl and cover to keep warm. Heat remaining olive oil in wok. Stir-fry onion, ginger and capsicum for 2 minutes or until onion has just softened. Return beef to wok with soy mixture and kale. Stir-fry for 2 minutes or until sauce has thickened and kale has wilted. Stir in sesame seeds and sesame oil.