Pat the fish dry with paper towel and place on chopping board. With a sharp knife, make four diagonal slits into one side of the fish. Turn fish and repeat on the other side. Combine salt with flour and coat the fish, dusting off the excess flour.
Heat oil in a hot wok until the surface seems to shimmer slightly. Carefully lower fish into the wok so it is completely covered by the oil, fry for 3 minutes, then carefully turn fish and fry for a further 3 minutes or until fish is lightly browned and just cooked through or the flesh is white through to the bone. If the flesh is still translucent, cook the fish for another minute or so. Carefully remove snapper and place on paper towel to drain. Transfer to serving platter.
To make the sauce, place the tamari, sake or white wine, mirin, ginger and onion into a small pot and bring to the boil. Reduce the heat and simmer gently for 5 minutes. To serve, pour the hot sauce over the fish.