Beat egg in a deep plate. Place flour on a second plate and breadcrumbs on a third plate. Drain mozzarella and cut each into 3 thick slices. Dip mozzarella slices first in flour, then in egg and then in breadcrumbs. Heat oil in a heavy pan and fry mozzarella slices over medium heat until golden brown. Rinse tomato, trim and cut into slices.
Serve mozzarella on warmed plates with tomato slices and garnished with basil.