Combine the vegetable broth, soy sauce, brown sugar, mirin, sriracha, and cornstarch in a small bowl. Mix until well combined.,
In a large skillet over medium heat, add the sesame oil. Once the pan is hot add the mushrooms. Sauté for 5-6 minutes.
Add the garlic and continue to sauté for another minute
Add the baby bok choy and cook for 2 to 3 minutes. It might look like a lot, but in will shrink as it cooks.
Add the sauce to the skillet and bring it up to a simmer and cook for 2 to 3 minutes. Stir so that the bok choy and mushrooms are well coated with the sauce.
If using tofu, add it in at the same time as you do the sauce.
Garnish with sesame seeds if desired, and serve.