Combine the first 8 ingredients in a saucepan, heat to a simmer, then simmer for 4 minutes. Season the fish fillets with salt and black pepper, then lightly dust with flour. Heat 1 tablespoon of the olive oil in a frypan over medium heat (until it shimmers). Place 1 tablespoon of butter in the pan, and as the butter starts to melt place a fish fillet, skin-side up, on top. Fry for about 3 minutes each side until cooked. Repeat with the other fillet. Place the fillets on plates and pour the sauce over. Serve with steamed bok choy, seasoned with fish sauce if desired.