Slice the flank steak thinly against the grain. In a bowl, combine with 1 tablespoon of soy sauce and cornstarch, and let marinate for about 5 minutes. Whisk sauce ingredients together in another bowl.

Heat skillet over high heat, spritz with cooking spray, add garlic and ginger, stir-fry for 30 seconds.

Add marinated flank steak in a single layer; cook for 2-3 minutes without stirring, then flip and cook for another minute until browned but tender. Remove from skillet.

Respray skillet before adding bok choy stems and red bell pepper; stir-fry for 2 minutes, then add bok choy leaves and cook for 1-2 minutes until wilted.

Return beef to the skillet, pour sauce over ingredients, and simmer for 1-2 minutes until sauce thickens.

Remove from heat, garnish with green onions and sesame seeds, and serve hot over rice.