1. Slice the knobbly roots and the skin from the celeriac, leaving as much flesh behind as possible, then give the celeriac a good wash. Chop it into small, 1cm-thick cubes. Set to one side.
2. Peel and finely chop the onion. Peel and grate or crush the garlic. Place a deep frying pan on a medium heat and drizzle in 1 tbsp olive oil. Add the onion, garlic and bacon lardons and fry for 5-8 mins, stirring often, till starting to soften.
3. Add the celeriac to the pan and pour in the hot stock. Simmer, lid off, for 10 mins till the celeriac is just tender. Stir occasionally.
4. While the celeriac is cooking, finely chop the flat leaf parsley leaves and stalks, then scoop them into a bowl. Chop the brie into small cubes and add to the bowl. Spoon in the crème fraîche and the horseradish relish. Crack in some black pepper and stir to mix. Set aside for later.
5. When the celeriac is cooked, turn off the heat. Using a slotted spoon, lift the veg into a large, clean bowl to cool, leaving any stock in the pan (you can discard the stock). When the mix has cooled for at least 30 mins, stir through the brie and crème fraîche mix.
6. Preheat your oven to 180°C/Fan 160°C/Gas 4. Line a large baking tray, or two smaller ones, with baking paper.
7. Remove 1 pack of pastry from the fridge and unwrap it. Lightly dust your work surface with flour and roll the pastry out into a large circle, approximately ½cm thick. Using a small side plate or large saucer, cut out 3 circles. Place a sixth of the bacon and celeriac mixture on to one half of one of the circles, leaving a 1cm-thick border around the edge. Repeat with the remaining 2 pastry circles and the celeriac mix.
8. Crack the egg into a bowl and beat. Brush the edges of the pastry with egg. Fold the pastry over the filling then pinch the edges together to seal and make a pasty. Repeat with the remaining 2 pasty rounds. Repeat steps 7-8 with the second block of pastry and remaining filling.
9. Carefully lift the pasties onto the lined baking tray then brush with the remaining beaten egg to glaze. Crack over some black pepper. Slide into the oven for 20-25 mins till the pastry is golden and the bottom of the pasty is crisp.
10. If you want to take the warm pasties on a picnic, wrap each pasty in a square of baking paper then wrap tightly in foil to keep them warm. They'll stay warm for a few hours. Alternatively, eat them straight away or let them cool and eat cold. The pasties will keep for up to 3 days in the fridge in an airtight container, or you can freeze them for up to 3 months. Defrost overnight and reheat in a medium oven for around 10 mins till piping hot all the way through.