Ground together bread (or breadcrumbs, or taralli), a few leaves of basil and a pinch of salt. Pour them in a bowl.
Prepare another bowl with the whisked egg and one more with the flour.
Cut each tomato in two, remove (use a small knife and spoon) the inner seeds and pulp (you can make some "pan con tomate") and sprinkle them with salt.
Fill them with a piece of mozzarella and a small (or cut) leave of basil. Close them: use a toothpick to keep the two sides together.
Dip each tomato in the flour, in the egg, in the bread, and back to the egg and to the bread.