Mix all marinade ingredients in a small bowl. Add the steak strips, toss to coat. Place in a plastic zipper bag and refrigerate overnight.
Heat a large skillet over medium-high heat. Add the oil and heat.
Mix the sauce ingredients in a small bowl; set aside.
Drain marinade from the steak. Discard marinade. Add the steak to the hot skillet and stir fry until it’s cooked and starting to brown.
Add the bok choy and bell pepper and stir fry for 2-3 minutes, depending on how crisp you like your veggies.
Stir the sauce so that the cornstarch is dissolved. Pour the sauce over the meat/veggie combo and stir until thickened. Cook for one additional minute so the cornstarch cooks through.