Put the puff pastry on a well-floured surface and bash it with a rolling pin. Usually you need to be quite delicate with puff pastry as it needs to puff up a lot, but for palmiers it only needs to puff up a little. Roll the pastry out into a rectangle about 30 x 35cm/12 x 14in and the thickness of a £1 coin. 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 towards each other to meet in the middle. Brush a little egg down the centre to stick the two halves together. Carefully lift into a large baking tray (making sure it will fit in your fridge first) and put in the fridge for at least 30 minutes to chill and harden.
Preheat the oven to 200C/400F/Gas 6.
Remove the roll from the fridge and, using a very sharp knife, slice it into 1cm/½in thick pieces. Lay each piece on the baking tray, cut-side up, and brush well with the beaten egg, then bake in the oven for 10–15 minutes until puffed up, crisp and golden-brown.
Remove the palmiers from the oven and leave to cool on the baking tray.
To serve, pile them high on a plate. These are great little canapés for waiting guests.