First, make the filling. Heat oven to 220C/200C fan/gas 7. Mix together 1½ tbsp oil with the cumin, cinnamon and vinegar, and rub the mixture all over the beetroot. Put the beetroot into a small roasting tin, season well, then cover with foil and roast for 20 mins.
Meanwhile, toss the celeriac with 2½ tbsp oil, the thyme, garlic and some seasoning in a second roasting tin. Separate out the slices so they cook evenly, then cover the tin in foil. In a third roasting tin, mix the sweet potato with the remaining oil, the smoked paprika and some seasoning, and cover with foil. Once the beets have cooked for 20 mins, add the celeriac and sweet potato to the oven alongside them, and roast all the veg for 40 mins further or until tender. Remove the thyme sprigs, squeeze the garlic cloves out of their skins and mash them in with the celeriac, then leave all the veg to cool. All the veg can be cooked the day before and kept in the fridge.
For the pastry, boil the kettle and use some coconut oil to grease a deep 20cm springform cake tin. Pour the flour into a bowl and add 1 tsp salt. Mix the coconut oil with 200ml boiling water, stir until melted (put it in the microwave if need be), then pour into the flour and mix with a wooden spoon to form a dough. Working as quickly as you can (it’s best to roll the pastry when it’s warm), cut off a ¼ of the dough and set aside under a tea towel. Roll out the rest to 0.5cm thick, then use it to line the cake tin, pressing the dough into the corners and leaving any excess pastry overhanging the sides. Don’t worry if the pastry breaks – it’s very forgiving, so you can patch it up as you go. Heat oven to 200C/180C fan/gas 6.
Now build the pie. Cover the base with chard leaves, then scatter over the semolina (which will absorb the beet juices), press in the beetroot, and season. Add another layer of chard, followed by the sweet potato, and season. Add a final layer of chard leaves, then top with the celeriac and season again.
Roll out the pastry you set aside to a thickness of 0.5cm to use as the lid. Put the lid on top of the pie and, using a fork, press together the overhanging pastry to create a crimped edge. Make a steam hole, then brush the top with a little almond milk mixed with a spoonful of oil (this will help to colour the pastry). Bake in the centre of the oven for 45 mins until the pastry is a deep golden brown. Leave to cool for 15 mins, then remove from the tin and serve in the middle of the table. Will keep for up to three days in the fridge (the pie is also delicious cold).