1. Preheat the oven to 200°C.
2. To make the roast celeriac-and-garlic cream, place the celeriac and garlic on a sheet of tin foil, dot with the butter and drizzle over the olive oil. Roast for 30 minutes, or until soft. Place in a blender with the cream and blend until smooth. Add some milk if it’s too thick. Season with salt and white pepper. Set aside and keep warm.
3. Mix the mashed potato, egg yolk, nutmeg, lemon zest and salt. Place a sheet of pastry on a lined baking tray. Cut out a circle using up most of the area. Place the mash on top and spread over the pastry, leaving a 2cm border.
4. Mix the egg and water and brush onto the border. Cut out the same size circle from the remaining sheet of pastry and place over the mash, sealing the edges with your fingers. Score the top with a sharp knife. Bake for 25 minutes, or until puffed up and golden.
5. Serve the pie with the celeriac cream.
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