Clean the fish thoroughly, rinse well, and dry with paper towels. Sprinkle some salt and pepper all over and set aside.
Prepare the sauce by combining onions (scallions), garlic, ginger, cilantro, tomato, lemon juice, pepper flakes, salt and pepper and 1 tbsp olive oil in a processor and pulsing a few times to create salsa-like consistency — fine chunk. Try not to turn it into puree.
With a sharp knife, cut the slits in the fish all the way to the bone, about 2-1/2? apart. Place fish on a sheet of aluminum foil if using the grill. Put the sauce into the slits on both sides of the fish first, then spread the remaining sauce evenly all over the fish
Drizzle the remaining olive oil around the fish and arrange potato quarters over it. Sprinkle some salt and pepper over the potatoes.
Put another sheet of foil over the fish. Grill (or bake) at 400°F for about 40 minutes, or until potatoes are done. Leave wrapped in foil until ready to serve.
While the fish is cooking, prepare the Bok Choi.
Melt butter in a wok or a large deep skillet over medium heat. Mince the garlic into the melted butter and saute for a minute until garlic is very fragrant. Take care not to burn it. Add Mirin and soy sauce.
Put Bok Choi into the skillet. Sprinkle some salt and pepper and toss a few times to evenly distribute the seasoning and sauce. Cover snugly with the lid, and cook on medium, shaking occasionally, until Bok Choi is wilted and cooked to your favorite doneness level. Some people prefer it slightly crisp, some — well cooked through. My favorite doneness level is somewhere in between.