MAKE THE PASTRY DOUGH: Measure the flour into the bowl of a food processor. Scatter the cold cubes of butter over the top. Process in short bursts until the mixture forms chunky breadcrumbs. Add the chilled sour cream and lemon juice and pulse again until mixture almost forms a ball. Remove onto a pastry mat or a sheet of glad wrap. Cover loosely and using your hands, flatten the pastry into a thick round disc. Cover in the pastry mat or glad wrap and refrigerate for at least 30 minutes.
MAKE THE CHEESE MIXTURE: In a medium-sized bowl, add all the ingredients and mix to combine. Refrigerate until required.
MAKE THE MUSHROOM MIXTURE: In a large frying pan heat the olive oil over medium-high heat. Add the mushrooms and spread them out evenly. Leave them for a minute or two before stirring, so the bottom ones start to brown. Add the bunch of thyme and stir the mushrooms every minute or so, until the mushrooms are nicely browned and all the moisture is gone from the pan. The mushrooms will start to caramelise slightly. This will take about 5-6 minutes.
Add the butter, onion, garlic, parsley, pepper and season lightly with salt. Cook for another 5 minutes, stirring frequently, until the onions have softened.
Add the sherry and stir to deglaze the pan. Cook until all the sherry liquid has evaporated and the mushrooms are caramelised and tender. Add the lemon juice, zest and mustard and stir through. Taste and season with a small amount of salt if required, remembering the cheese will be slightly salty too. Remove the thyme sprigs and discard. Set mixture aside to cool to room temperature before assembling the galette. To speed up the cooling process, you can spread the mushroom mixture onto a tray.
EGG WASH: In a small bowl, whisk together the egg and milk. Set aside.
ASSEMBLE THE GALETTE: Preheat the oven to 200°C (400°F). Remove the dough disc from the fridge. Dust a work surface with flour (I use a pastry mat or a sheet of baking paper but you can use the kitchen bench) and lightly dust the dough disc with flour. Take a rolling pin and roll out the dough to form a 30cm (12inch) round. If the dough is quite firm, let it rest on the bench for 5 minutes. Start from the middle of the dough disc and roll out, rotating the dough around as you go, so it doesn’t stick to the work surface. Continue to lightly dust the dough and work surface with more flour if required. Once rolled, carefully transfer the dough round to a baking paper lined oven tray. At this stage, I like to chill the dough in the fridge for 15-20 minutes, as rolling it out causes the pastry to warm up and soften.Tip: By using a pastry sheet or baking paper, this enables you to easily transfer the rolled out dough to the oven tray. Use 2 pieces large enough to accommodate a 30cm circle of dough, one underneath the dough and one on top and place the dough disc right in the centre. By rolling the dough out between the sheets of baking paper you won’t need much flour (if any). Once the dough is rolled out, you simply peel off the top sheet and transfer the bottom sheet with the rolled out dough straight onto the oven tray. Once the mushrooms have cooled to room temperature, add half of them to the cheese mixture.Remove the now chilled 30cm dough circle from the fridge. Spoon the cheese mixture into the middle of the dough, spreading it out evenly, leaving a 3-4cm border of dough on all sides. Spoon the remaining half of the mushrooms over the top of the cheese mix and spread out evenly. Carefully fold the dough sides up and over the top of the filling, keeping the shape of the circle as best you can and leaving the centre exposed. Don’t be too precious with the folding, as it is a rustic looking tart (see photos).Brush the pastry with egg wash. Sprinkle with a little extra parmesan cheese and season the pastry very lightly with salt.Sprinkle extra parsley over the mushrooms (optional) and bake in the preheated oven for 35-45 minutes or until the pastry is golden in colour. Remove from the oven and allow to cool on the tray for 5-10 minutes before serving. Garnish with extra thyme leaves if desired.