Beef wellington

Name: Beef wellington
Cook time: 2 h 30 min
Total time: 2 h 30 min
Cuisine: British
Ingredients: a good beef fillet (preferably Aberdeen Angus) of around 1kg/2lb 4oz; 3 tbsp olive oil; 250g/9oz chestnut mushroom, include some wild ones if you like; 50g/2oz butter; 1 large sprig fresh thyme; 100ml/3.5 fl oz dry white wine; 12 slices prosciutto; 500g/1lb 2oz pack puff pastry, thawed if frozen; a little flour, for dusting; 2 egg yolks beaten with 1 tsp water