How to Make It
Preheat the oven to 375°. In a small skillet, cook 3 tablespoons of the butter over moderately high heat, stirring, until the milk solids are golden, 3 to 5 minutes. Transfer to a small bowl; let cool completely. Using a fork, mix in the chile oil, peppercorns and remaining 3 tablespoons of butter. Season the Sichuan butter with salt and refrigerate until spreadable, 10 minutes.
Grease a rimmed baking sheet with canola oil. Using a knife, make 4 shallow slashes on each side of the fish. Season with salt and pepper and transfer to the baking sheet. Roast for 15 minutes, then spread the Sichuan butter on top of the fish. Roast for about 20 minutes longer, basting occasionally, until the thickest part of the flesh flakes with a fork.
Meanwhile, in a large skillet, heat the 1 tablespoon of canola oil. Add the leek and garlic and cook over moderately high heat, stirring, until just softened, 3 minutes. Add the bok choy and a pinch of salt. Stir-fry until the bok choy is just wilted, 3 to 5 minutes. Remove from the heat and stir in the vinegar. Season with salt and pepper.
Using a large, thin spatula, transfer the fish to a platter and spoon any pan juices on top. Pile the bok choy around the fish and drizzle with the sesame oil. Serve right away.