Defrost the pastry squares at room temperature (it should take approximately 10 minutes). Preheat the oven to 220°C / fan 200°C / gas 7.
Using a small sharp knife, lightly score a border around the pastry square 1cm from the edge. Spread with the tomato purée and then sprinkle with the dried herbs, being careful not to go over the border. Top with overlapping slices of tomato, cheese and ham, tearing the ham into pieces and folding it as you put it in place.
Whisk the egg yolk with 1 tbsp cold water and then brush the border with the egg mixture. Place the squares on a baking tray and cook in the oven for 16-18 minutes or until golden brown. Lightly dress with the rocket and serve.