Dice the tofu into 1 inch cubes and arrange in a large, shallow earthenware casserole. Add boiling water to cover and place over medium heat.
Combine, in a sauce pan, the soy sauce, sake, sugar, bonito shavings, and 2/3 cup water. Heat on medium-high and simmer for 3 minutes then turn off heat.
When the water the tofu is cooking in starts to boil, remove the casserole to the table. Pour 1/4 cup of the prepared sauce into individual saucers and place before each guest, together with a saucer of chopped scallion and grated ginger. Cut each lemon in half and serve one half to each guest.
Each guest helps themselves, transferring the tofu to an empty saucer. Have each guest squeeze the lemon into their prepared sauce and dip the tofu into the sauce then into the scallion and ginger, as preferred, and enjoy.