Preheat the oven to 200°C/400°F/gas 6.
Wash, trim and dice the leek, core and dice the apple, then pick the thyme leaves.
Combine the pork, apple, thyme leaves and mustard seeds in a bowl. Season and set aside
Roll out the pastry to 1cm thick and 34cm x 30cm. Halve lengthways and place a strip of mince down the centre of each.
Brush the edges with beaten egg, roll up and seal. Brush with more egg, then cut each strip into 3 rolls.
Score the tops and bake for 20 minutes, or until golden and cooked through.