Let all three fillings cool completely before you assemble - warm filling melts the pastry and steams it from the inside, so this is the single biggest factor in a crisp Wellington. Check that your puff pastry is dairy-free if you want to keep the recipe fully vegan, as many brands are made with butter. Leftovers keep in the fridge for up to 3 days; reheat in the oven at 180°C (350°F) for about 10 minutes to bring the crust back.