Over high heat, bring a large stockpot filled with salted water to a boil. Meanwhile, prepare an ice bath, and set aside. Add bok choy to the boiling water. Blanch until just tender, about 1 1/2 minutes. Immediately transfer bok choy to the ice bath to stop the cooking process. Drain, and set aside.
Place ginger and shallots in a medium mixing bowl. Whisk in oyster sauce and vinegar. Whisk in 4 tablespoons canola oil, the soy sauce, lime juice, and cayenne pepper. Set aside. (The recipe can be prepared up to this point 2 hours ahead.)
In a large saucepan over high heat, bring 1/4 cup water to a boil. Whisk in butter, and lower the heat to medium-high. Season bok choy with salt and pepper, and add it to the pan. Cook until heated through, about 2 minutes.
Divide the remaining 2 tablespoons canola oil between 2 (10-inch) nonstick skillets. Place skillets over high heat until oil is just smoking. Season both sides of the grouper with salt and pepper. Place 2 grouper fillets in each skillet, and cook until the fish is browned on the bottom side, about 3 minutes. Turn, and saute about 3 minutes more, or until a metal cake tester inserted into the center of the fillets feels hot when touched to your lip. Remove from heat, and set aside.
Using a slotted spoon, transfer the bok choy from the saucepan, and arrange it in the center of 4 dinner plates. Top with the grouper. Whisk the sauce lightly, and spoon it around the bok choy. Sprinkle sesame seeds over the sauce, and serve immediately.
(c)2003 Martha Stewart Living Omnimedia, Inc. All Rights Reserved.