Wash sage, shake dry and pluck leaves from the stems. Drain mozzarella and cut into 8 slices. Bark the toast. Coat the edges of the toast with cold water.
Cover 4 slices of toast with 2 slices of mozzarella and 2 sage leaves each. Season well with salt and pepper. Cover with remaining toast slices and press the edges firmly together. Whisk eggs and milk.
Season with salt and pepper. Turn filled toast first in flour and then in the egg mixture. Heat the oil in a pan and fry the toast for about 2 minutes on each side until golden brown. Remove the toast and let it drip off on kitchen paper.
Add the rest of the sage leaves to the hot frying fat and fry for about 1 minute. Halve the toast and serve together with the sage.