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.