Heat the butter and oil in a large frying pan and fry the sweet potatoes and cook over a low heat for 8-10 minutes until just beginning to soften. Stir in the mushrooms, cooked turkey, garlic and half the thyme and cook for a further 4-5 minutes.
Preheat the oven to 180°C/Gas mark 6. Pour the brandy over the turkey mixture and allow to bubble for 1 minute, then add the stock and cream and warm through gently. Remove from the heat and stir in the Stilton.
Unroll the pastry. Cut 4 pastry lids slightly larger than the 400ml dishes you are using. Divide the filling between the 4 dishes. Wet the rim of the dishes with water. Lay the pastry circles over the top and press to the rim firmly. Brush with beaten egg and scatter with rock salt and remaining thyme. Bake for 20 minutes until golden.