In a mixing bowl, mix together the leek, ricotta, breadcrumbs, cheddar, mustard, beaten egg, and season.
Cut the puff pastry in half to make two rectangles.
Place half the mixture in a sausage shape down the middle of the long side of one rectangle, then roll up and seal edges.
Repeat with the rest of the mixture and pastry.
Cut each log into 7 pieces, then place on lined baking sheet.
If you beat the egg in a bowl/cup, use the tiny bit that’s left - with your hands – to spread on top of the rolls (otherwise use a little milk or water) then sprinkle over sesame seeds.
Place in the oven and bake for 20 minutes, until golden.