In a medium bowl, combine beef strips with soy sauce, sesame oil and corn starch. Let sit for at least 5-10 minutes while you prepare the bok choy.
In a skillet or wok, bring 1 cup of water to a boil. Add bok choy. Cover and cook on medium-high heat until bok choy stems are tender when pierced with a fork. Add about 1/2 Tbsp to 1 Tbsp of oyster sauce (more or less to taste) and toss the bok choy. Transfer bok choy to a plate.
Drain off any excess water from the skillet and make sure the skillet is dry.
Heat skillet on medium-high heat. When skillet is hot, add 1 T cooking oil.
Add beef strips to skillet and let cook for 1 minute, undisturbed. Then stir-fry beef strips for another 3-4 minutes, depending on desired level of doneness. Add remaining oyster sauce to taste and combine.
Place bok choy over rice. Top with beef and sprinkle with sesame seeds.