Remove any sinew from the outside of the pork fillet and then pre-cook - either pan fry for approx. 20 minutes or cover and cook in the oven for around the same length of time. Set aside to cool.
Sauté the onion, garlic, thyme and mushrooms until they look like a dark brown mush, then cool. Stir in the crème fraîche and season. It's important to cool the ingredients before they come into contact with the pastry, or it won't roll up properly.
Lay out the pastry and trim to fit the length of the pork fillet, then spread evenly with the mushroom mixture. Place the fillet at one end and carefully roll it up.
Place on a baking tray with the seam side facing downwards.
Gently slash through the pastry in places (to allow steam to escape), brush over with beaten egg and bake in the oven at gas 5/190°C for around 35 mins, until the pastry is golden brown.