Make chia egg by mixing together 1 tbsp of ground chia seeds with 3 tbsp of water. Set aside (if using a regular egg, omit this step and see step 3).

With a sharp knife, chop finely the onion, garlic, mushrooms and leek. Heat 3 tbsp of oil in a large frying pan. Add the garlic and onion and fry under medium heat until soft. Next add the mushrooms, leek, parsley, thyme and Dijon mustard. Stir well, cover with a lid and cook under medium heat for 10 mins.

If using chia egg: add spinach leaves and chia egg into pan. Stir well, cook covered under medium heat for another 5 mins. Mix again. Cook uncovered for 5 mins or so until all juices in the pan have evaporated. If using egg: add spinach leaves and cook uncovered until all juices in the pan have evaporated. Set aside to cool down then add beaten egg.

Preheat the oven to  200C/180C fan/gas 6. Next prepare the pastry: roll out the pastry sheet horizontally in front of you on baking paper into a large rectangle of 24cm height x 37 cm width. Mark and divide the pastry into 3 horizontal rectangles (8cm height) by gently pressing knife on the pastry. Do not cut though the pastry. Mark a 2cm edge on both the right and left side of the pastry, keep these areas free. With a sharp knife, start to cut 2 to 3 cm diagonal strips along the top and bottom rectangles on the pastry.

Pile mushroom mixture in the middle rectangle, packing it tightly. Leaving the 2cm edge on both the right and left side free. Make sure the mushroom mixture is as dry as possible otherwise the pastry will get too wet.

Start by folding the left hand side edge of the pastry to close the pie. Then start plaiting by folding in the top strip down and bottom strip up on top of the other strip; repeat until you reach the right hand side edge. Fold the right hand side edge to close the pie. Don’t worry if the plait is not perfect as long as most the mixture is covered. 

Brush some melted (dairy free) butter all over. Place in the oven and cook for 20-25 mins until golden brown. Once done, leave to rest for 10 mins before serving.

If you prefer a vegetarian option, you can use an organic egg instead of a chia egg. Make sure you mix it at the end so it does not cook in the pan.