Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and ginger, stirring for about 30 seconds until fragrant.
Add the sliced mushrooms and red bell pepper, cooking for 3-4 minutes until they start to soften.
Stir in the chopped bok choy and cook for an additional 2-3 minutes until wilted.
Pour in the soy sauce, oyster sauce (if using), and sesame oil, stirring to combine.
Add the cornstarch mixture to the skillet, stirring until the sauce thickens, about 1 minute.
Season with salt and pepper to taste. Remove from heat and garnish with sesame seeds if desired.