Heat a drizzle of oil in a large non-stick pan. Add the sliced leek and fry on a medium heat for 5 minutes until soft.
Place the chicken mince in a large mixing bowl with the breadcrumbs, Rosemary, cubed cheese, fried leeks and season with salt and pepper.
Use your hands to give it a really good mix, until the breadcrumbs are absorbed and the ingredients are evenly spread.
Roll out the sheets of puff pastry and cut them in half lengthways until you have 4 equal sized pieces.
Heat the oven to 200C fan.
Divide the chicken mixture into 4 and spread it evenly down the middle of each of the pastry pieces. Roll it up and gently flatten the top down a little.
Cut each of the pastry rolls in 3 even pieces so that you have 12 chicken rolls. Place them on a lined baking tray. Cut some slits in them and brush with beaten egg.
Bake in the oven for 25 minutes, or until golden brown and cooked through.
