In another pan, fry the onions the chopped celery and salt until onions begin to brown. Stir in the mushroom, garlic and thyme. Cook for 2-3 minutes on medium heat.

Reduce heat and sprinkle with flour and half of lemon zest, use a whisk to mix it in, cook for 1 minute. Combine the vegetable stock with the cream and whisk it into the mushroom mixture little by little, stirring continuously until it thickens, it will take about 8-10 minutes.

Butter a 9-10" pie dish sprinkle with lemon zest and little flour. Roll out half of your pastry (separate recipe), and transfer it into the dish. Gently pile the cooled filling onto the pastry. Now, roll out the other one and place it on top of the filling pressing in the edges.

Bake in a pre-heated fan assisted oven, on the middle shelf for 1 hour on190 C. Allow to cool a little before slicing it. If any left over, rest assured it will keep in the fridge for up to a week. Enjoy!