Compose the sandwich
Remove the crust from the bread slices, cut the slices in half, then cover half of the slices with the mozzarella slices; cover them with the remaining bread slices, and press with the hands to make them adhere.
Pass the sandwiches in the flour
Put a couple of tablespoons of flour on a plate, and pour some water in a bowl. Holding tight the sandwiches, to prevent them from open, pass the edges of the sandwiches in the flour, pressing carefully.
Pass the sandwiches in the water
Immerse the edges of the sandwiches in the water, trying to soak no more than 0.2 inches of bread; this step is important to create a sort of "glue" that will seal the mozzarella inside the bread.
Pass the sandwiches in the egg mix
In a soup plate beat the eggs with the milk and a pinch of salt, and dip the sandwiches one by one, turning them on both sides: the bread must absorb the egg mix uniformly, but remaining compact.
Fry the sandwiches and serve
Fry the sandwiches, few at a time, in a pan plenty of hot oil, 2-3 minutes each side, turning them using a tong; drain them when they start to get brown, lay them on absorbent paper, and serve the sandwiches hot.
Some tips: mozzarella
Use cow-milk mozzarella: it is more compact than the buffalo-milk mozzarella. It is better if the mozzarella is not fresh: if the mozzarella has one or two days it will be drier, perfect to cook.
Some tips: bread
Choose a loaf of stale homemade bread that has a couple of days: it will contain the mozzarella better, without breaking during the cooking, and it will be crunchier. Cut it in slices of no more than 0.4 inches, using a serrated blade. Avoid using the pan loaf, even if it is very convenient because it is already cut into regular slices: is unsavory and too porous.