Heat the oven to about 200 degrees Celsius (or whatever it recommends on your pack of pastry). Line a baking tray with non-stick paper, then unroll the defrosted pastry directly onto it. Cut into it to a depth of about halfway down all around, about 3cm from the edge, to form an inner rectangle. Prick the inner area all over with a fork and then spread with the pesto, making sure none goes onto the border.
Slice your tomatoes, guesstimating how thick they need to be in order to cover the inner area with them - the more you have, the thicker you can afford to have them, and arrange them on top of the pesto, again not going over the border. Then grate a little cheese over the whole thing.
At this point you can get all fancy with the border, scoring fancy patterns into it, egg washing it, whatever floats your boat, only then it isn't the World's easiest any more, is it? And even the keenest cooks have lazy days.
Finally stick it in the oven until the border is puffed up and golden, by which time the tomatoes will be cooked.
I'm linking this to Food on Friday where this week's challenge is tomatoes