Preheat the oven to 400°F.
Make a large pouch from aluminum foil and place the cod inside. Top with slices of butter and the garlic, evenly divided.
Tuck the bok choy around the llets. Season with the salt and pepper.
Close the pouch with the two ends of the foil meeting at the top, so the butter remains in the pouch.
Place the sealed pouches in a baking dish. Put the dish in the preheated oven, and bake for 15 to 20 minutes, depending on the thickness of the llets.
Remove the dish from the oven and check the llets for doneness.