In a large non-stick frying pan, heat a little olive oil then sauté the mushrooms until browned and cooked through. Add the garlic and thyme and fry for another 30 seconds before removing from the heat.
Shred the chicken, discarding any bones and skin, and place in a bowl. Add the mushrooms, cheese sauce, crème fraîche and lemon juice and mix well. Season to taste with salt and pepper.
Place the filling in a pie dish/ovenproof dish of your choice then top with the pastry.
Trim the edges and crimp the pastry before brushing with the beaten egg.
Place in the oven and allow to bake for 15-20 minutes until the pastry is golden brown and puffed and the filling has heated through.
Remove from the oven and serve with vegetables of your choice.