Preheat the oven to 180?.
Unroll the puff pastry sheet. Use an egg cookie cutter to cut 16 egg shapes.
Lay 8 of the egg shapes on an ovenproof cookie sheet. Put 1 tsp of Boursin in the centre of each one.
Cut each of the tomatoes in half. Take one half of tomato and lay it cut side down in the middle of one of the remaining egg pastry shapes. Use a sharp knife to cut the pastry around the tomato, then take the tomato away. You should have a tomato shape piece of pastry cut out of the centre (you can discard this).
Now, lay that half of tomato cut side down on top of the Boursin on one of the egg shapes. Take the other egg shape, with the centre piece discarded, and place it on top. It should now fit snugly with the tomato coming out of the centre. Press down the edges firmly.
Repeat with the other egg shapes, until all 8 tartlets are complete.
Brush the top of the tartlets with olive oil and sprinkle with sesame seeds.
Put in the oven for 15 minutes or until golden. Remove from the oven and allow to cool before eating.