Filling: Peel and slice the sweet potato. Peel the garlic clove. Place in an overproof dish. Sprinkle with the cumin, allspice and cinnamon. Drizzle with the oil. Cover with foil and cook until tender. This will take at least 30 minutes.
Meanwhile, chop the onion finely. Heat 2 tbs of oil in a saucepan and cook gently until really soft with a lid on. If it is starting to brown a lot, add some water. This will take at least 10 minutes.
When the sweet potatoes and onion have cooled add to a blender or food processor and mix to a puree. Set aside.
Whilst the oven is still hot, toast the seeds. Put the seeds into a dish in a single layer and bake for about 7 minutes until golden. Set aside.
Pastry: First make the pastry. Rub the Trex and nut butter into the flour and salt. Add enough water so that you can form a ball. Nut butters do vary, so the amount of water you may need to add will vary too.
Roll out to fit your 20 cm tin or ring. Gently place in the tin, pushing the pastry into the corners.
Trim the edges and place in the fridge for at least 10 minutes to firm up.
Preheat your oven to 190°C.
Line the uncooked pastry case with baking parchment. Add ceramic baking beans (or as I do dried beans). Cook the pastry for 10 minutes (blind bake).
After 10 minutes, take the baking parchment and beans out of the pastry case. Bake for another 5-7 minutes until it is golden brown.
Assembly: Taste the sweet potato, onion and garlic mixture and add salt and pepper to taste.
If your oven is not currently on, heat to 190°C.
Fill the pastry case with the sweet potato and onion filling. You may find you have a little more mix than needed. If so, you can serve separately.
Use a fork to make grooves across the top and drizzle with oil.
Put the pie in the oven to heat through for about 15 minutes. Sprinkle with the seeds before serving.
