Image credit to: Boston Organics
Combine the soy sauce, sake, sugar, ginger, garlic, and scallions in a small saucepan. Bring to a simmer over high heat stirring until the sugar dissolves. Reduce to a bare simmer and cook until sauce is syrupy and reduced to about 1/2 cup, about 20 minutes. Strain and discard solids. Reserve sauce.
Toss bok choy with olive oil and season with salt and pepper. Place on hot side of grill, cut side down and cook until lightly charred, about 45 seconds. Flip and cook until second side is charred, 45 seconds longer. Transfer to cooler side of grill, cover, and continue cooking until tender all the way through with a light crisp bite, 1 to 3 minutes longer. Transfer to a large plate, drizzle with sauce, and serve.