Heat barbecue grill to low. Lay 2 sheets of foil on top of each other on a flat surface. Put 1 fillet in centre. Add a little of the peppercorns, ginger, shallot, soy, fish sauce and lime juice, then wrap up tightly. Repeat to make 4 parcels.
Add parcels to grill and cook for 12 minutes or until fish is just firm. Put fish and peppercorn mixture on serving plates. Drizzle over parcel juices, then oyster sauce. Serve with bok choy on the side.