Heat 1 tablespoon olive oil in a pan and fry onion till soft, about 5 minutes. Add garlic and thyme. Fry gently for a few more minutes, then transfer to a 1.2 litre ovenproof dish.
Heat 1 tablespoon olive oil and brown pork for a few minutes in the pan, then add to dish.
Melt butter in the pan and cook mushrooms until softened, about 6 to 8 minutes. Add to pork and onions and mix in the flour.
In a jug, mix chicken stock with double cream and lemon zest. Season well and pour into dish.
Unroll pastry and place on top of the dish. Trim the edges and use the pastry trimmings to decorate the top of the pie. Brush with milk and bake in the preheated oven until golden brown, about 30 to 40 minutes.