Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and grated ginger, stirring for about 30 seconds until fragrant.
Add the sliced shiitake mushrooms and cook for 3-4 minutes until they begin to soften.
Stir in the chopped bok choy and sliced red bell pepper, cooking for an additional 3-5 minutes until the bok choy is wilted and tender.
Pour in the soy sauce and sesame oil, stirring to combine.
Add the cornstarch mixture to the pan, stirring quickly to thicken the sauce.
Season with salt and pepper to taste.
Remove from heat and garnish with sesame seeds if desired.