Put the puff pastry on a well-floured counter and bash it with a rolling pin. Usually, we need to be quite delicate with puff pastry, as it needs to puff up a lot, but for palmiers it needs to puff up only a little. Roll the pastry out into a rectangle about 12 by 14in (30 by 35cm) and 1/8in (3mm) thick. Spread the tomatoes over the puff pastry and sprinkle over the rosemary.
With the shortest end facing you, take both long edges of the pastry and roll them toward each other to meet in the middle. Brush a little egg down the center to stick the two halves together. Carefully lift onto a large baking sheet, making sure it will fit in your refrigerator first, and put in the refrigerator for at least 30 minutes to chill and harden.
Preheat the oven to 400 degrees F (200 degrees C).
Remove the roll from the refrigerator and, using a very sharp knife, slice it into 1/2in (1cm) thick pieces. Lay each piece on the baking sheet and brush well with the beaten egg, then bake in the oven for 10 to 15 minutes until puffed up, crisp, and golden.
Remove the palmiers from the oven and let cool on the baking sheet. To serve, pile them high on a plate. These are great little canapes for waiting guests.