Preheat the oven to 200C/400F/Gas 6. Grease two holes in a muffin tray.
Roll each slice of bread with a rolling pin until flattened. Using a 10cm/4in biscuit cutter, cut out two circles. Cut out a further two circles with a 7cm/3in cutter.
Press the larger bread circles into the holes in the muffin tray.
Mix the tomatoes, tuna, mozzarella and basil together in a bowl. Divide the mixture between the two bread cases. Top with the smaller circles of bread and press down to seal the edges.
Drizzle with a little oil and bake in the oven for 12-15 minutes, or until golden-brown.