Fill a wok with salted water, and bring to a boil. Boil Brussels sprouts for 2 minutes, pour into a colander, rinse with cold water, and drain well. (Don’t worry if some of the leaves fall off—they’ll be lovely.) Dump sprouts onto a kitchen towel, lay another towel on top, and blot dry. Let them sit while the mushrooms cook. Wipe out wok.
Heat wok over high heat (keep the heat really high during all of the stir-frying) for 1 minute, and swirl in 1 tablespoon oil. Add mushrooms and garlic, and stir-fry for 3-4 minutes, until browned and fragrant. Scoop into a bowl.
Heat remaining 2 tablespoons oil, and swirl around wok. Add shallots and cook for 1 minute, stirring constantly, until golden brown. Add Brussels sprouts to pan with a pinch of salt and pepper, and cook, tossing constantly, for 5-7 minutes, until they’re charred and tender, but not yet mushy.
Reduce heat to medium, and stir the mushroom mixture back in. Pour in chile sauce and fish sauce. Toss to coat vegetables, sprinkle with cilantro, and add additional salt and pepper, if desired. Serve with Sriracha (if using).