Preheat oven to 425°F.
Combine the chicken, beaten egg, and sesame seeds in a medium bowl, tossing to coat.
In a separate bowl, combine the flour and cornstarch. Dip the chicken pieces into the flour/cornstarch mixture and then transfer them to the baking sheet, making sure to not overcrowd the pan. Bake for 5 minutes.
Flip the chicken with tongs and add the bok choy to the pan, cut side down. Drizzle the chicken and bok choy with the olive oil and bake for an additional 10 minutes.
While the chicken and bok choy are cooking, whisk the teriyaki sauce, water, sesame oil, sesame seeds, and cornstarch together in a small bowl. Microwave for 2 minutes, stopping to whisk halfway through. The sauce should thicken slightly to the point where it can coat the back of a spoon. If not thick enough, whisk again and microwave an additional 30 seconds.
Once the chicken and bok choy have baked for 10 minutes, pour the sauce over the chicken and toss to combine.
Bake another 5 minutes, or just until the sauce is beginning to caramelize and the chicken is cooked through.
Serve with white or brown rice and garnish with sesame seeds and sliced green onions.