Heat the oil in a large skillet or wok over medium heat. Add the shrimp and cook, stirring frequently, for 2-3 minutes, until it is somewhat opaque.
Add the mushrooms, bok choy, garlic powder, tamari, and lemon juice and zest. Continue to cook, stirring frequently for several minutes until the bok choy has wilted and the shrimp is completely cooked through.
Crack the egg over the stir-fry and lightly stir through. Remove from heat and let sit for a minute or too, allowing the egg to fully cook through. Serve.