To make the pastry, put both flours into the bowl of a food processor with the butter, thyme leaves and ½ tsp fine sea salt. Pulse for a few seconds until the mixture resembles sandy rubble, then add the whole eggs and 2-3 tbsp cold water. Pulse again until it comes together into a ball, then transfer to a lightly dusted work surface and knead briefly until smooth. Wrap and chill for 30 mins.
To make the filling, heat the oil in a frying pan over a low-medium heat, and cook the onion for 30 mins until sticky, golden and caramelised. This will take some time, but it will add depth and sweetness to the pie. Add a splash of water if the onions start to darken too quickly or catch.
Add the parsnips and 250ml water. Cook for 15 mins, stirring often until the water has evaporated and the parsnips are tender and just starting to colour. Remove from the heat and leave to cool.
Mix the eggs and crème fraîche together in a large bowl until smooth, then fold in the feta, thyme, butter beans and the veg mixture. Add the mace, lemon zest, ½ tsp black pepper and a little salt.
Cut off two-thirds of the pastry and roll it out on a lightly floured work surface to a thickness of 3-4mm. Use it to line a 23cm cake tin or round pie dish (it should be about 7cm deep), leaving some pastry overhanging. Spoon in the filling, levelling the top.
Roll out the remaining pastry until it’s large enough to cover the top of the pie. Brush a little of the egg yolk around the edge of the pie, then lift the pastry lid over the top and press the edge to seal. Trim any excess pastry with a sharp knife. Make a steam hole in the middle of the lid, then glaze with the rest of the egg yolk. Sprinkle over the dried herbs, then chill for 30 mins. Will keep in the fridge for up to 24 hours.
Heat the oven to 180C/160C fan/gas 4. Bake the pie for 50 mins until the pastry is golden, crisp and dry. Leave to rest for 15 mins before cutting and serving.