Lay the mozzarella and tomato slices on separate paper towels for 10 minutes to remove surface moisture.
Slice the focaccia horizontally and spread the pesto over both cut faces.
Layer the mozzarella and tomato across the bottom half, then season them with the salt and black pepper.
Add the basil leaves and drizzle the balsamic glaze in thin lines over the filling.
Close the focaccia, wrap it in parchment and press it under a skillet for 10 minutes.
Unwrap the sandwich and cut it into 4 equal rectangles.