Preheat oven to 200 C / Gas 6. Put the sweet potatoes in the centre of the oven on a baking tray. Cook for around 20 minutes, or until soft.
Heat the oil in a frying pan. Fry the lamb mince and white onion in the vegetable oil, seasoning with the coriander, cinnamon, chilli powder and cumin.
Put the cooked mince and onion into a large bowl. Remove the insides of the sweet potatoes from the skin and place in a separate bowl.
Mash the sweet potatoes with a knob of butter. Not too much, as the sweet potato will already be soft. Season with salt and pepper.
Add the sweet potato, spring onions and egg to the mince mixture, mixing well. Melt around 2 tablespoons of butter in a small pan.
Place the filo pastry on a work surface. Take a sheet of the pastry and put a damp tea towel on the other sheets (filo pastry dries out very quickly).
Brush the sheet of pastry with some butter and add a sprinkle of cinnamon and chilli. Place another sheet on top, and repeat the brushing and seasoning. Do this for a third time.
Cut the three piled sheets of pastry diagonally to make two triangles. Put a little dollop of the lamb mixture into one corner of both pastry triangles. Fold the pastry over the dollop of mixture, pinching the sides and brushing with butter.
Repeat steps until the lamb mixture runs out. Put all of the brik on a baking tray.