How to Make It
Whisk flour, sugar, baking powder, baking soda, and salt in a large bowl. Whisk milk, egg yolk, ginger, oil, 2 tablespoons butter, 1 tablespoon tamari, and ⅓￼ of the garlic in a bowl. Add milk mixture to flour mixture; stir until just combined. Beat egg whites with an electric mixer on medium-high until stiff peaks form, about 3 minutes. Gently fold into batter.
Heat 1 tablespoon butter in a 10-inch nonstick skillet over medium. Pour in batter. Reduce heat to medium-low and cook, covered, until batter is bubbling and edges are set, 8 to 10 minutes. Using a metal spatula, flip pancake. Continue cooking, uncovered, until underside is golden and a toothpick comes out clean, 4 to 6 minutes. Transfer to a serving platter.
While pancake cooks, heat 1 tablespoon butter in a large skillet over medium. Add scallion whites and remaining garlic and cook 2 minutes. Add mushrooms and bok choy stems; cook, stirring, until mushrooms are golden, about 3 minutes. Stir miso and remaining 1 tablespoon tamari and 1 tablespoon butter into skillet until smooth. Add bok choy leaves; cook 1 minute more. Remove from heat.
Serve pancake topped with mushrooms and scallion greens, with sriracha on the side.