Soak two salt-packed anchovies in cool water for a few minutes. Peel the filets off of the bones.
Build a sandwhich in this order: quarter-inch sliced fresh mozzarella, quarter-inch sliced tomato, salt and pepper, anchovy filets, torn basil.
In a cast-iron or non-stick pan, gently cook the sandwich in olive oil, so that the cheese melts and the anchovies soften as the bread browns. Cook with cheese-side down first, so that the cheese melts and holds the sandwich together before the flip. You may find that weighting down the sandwich as it cooks helps with even heating and browning.