Set oven to Gas 7, 220°C, 425°F. Unroll pastry and cut into about 18 small circles using a 8cm (3½in) fluted pastry cutter.
Spread each with mustard, leaving a 1cm (½in) border. Cut tomatoes into slices and place 1 slice on top of each piece of pastry and put on a baking tray.
Bake for 12-15 minutes until risen and golden brown.