1Place milk, flour, and butter in a pan and whisk until melted and thickened (about 5 mins). Stir in cheeses and set aside.
2Preheat oven to 200°C fan/425°F. Place pastry on a baking tray lined with parchment paper; drizzle with olive oil and sprinkle with thyme, salt, and pepper.
3Scatter mushrooms, leeks, and shredded chicken evenly over the pastry, leaving a 1cm border.
4Pour half the cheese sauce over the chicken, leeks, and mushrooms.
5Drape puff pastry over the filling; press edges to seal. Score the top with a knife and brush with egg.
6Bake for 30 minutes until golden and puffy. Let sit for 5 minutes, then carefully flip onto a chopping board, peel off parchment paper, and bake for another 5 minutes to brown the top and bubble the cheese.