Clean the fish by scraping off the scales, removing the guts and gills, and cutting off the edges of the tail and fins.
Score the fish by making 3 to 4 diagonal incisions on both sides, about half an inch deep.
Season with salt and pepper, rubbing the seasonings into the incisions.
Finely mince the garlic.
Cut the onion into very thin slices.
Cut the ginger into matchsticks.
Take two large pieces of aluminum foil. One should be about six inches longer than the fish and the other should be three-fourths longer than the first piece.
Brush a tablespoonful of oyster sauce on each side of the fish.
Lay the fish on the smaller piece of aluminum foil.
Scatter the onion, garlic and ginger over the whole length of the fish.
Cover with the longer piece of foil, pulling up the center to create a "tent". Gather the edges of the top and bottom pieces of foil and fold inward two to three times. Do this with all four sides of the foil.
Transfer the fish in the foil on a baking sheet and cook in a preheated 180C oven for 20 to 25 minutes.
Take the fish from the oven, unroll the edges and carefully peel off the top piece of foil.
Heat the sesame oil in a small pan until smoking and pour over the fish.
Sprinkle the snipped cilantro and parsley over the fish and serve at once.