Preheat your oven to 200°C/400°F/gas 6. Wash the sweet potatoes and bake them, in their skins, in the preheated oven until cooked and soft. Allow to cool (leaving the oven on), then peel and mash them.
Bash up 1 teaspoon of the coriander seeds and the chillies in a pestle and mortar. Stir with the cinnamon into the mashed sweet potato and season well. Fold the spring onions and beaten egg into the mashed potato.
Gently melt the butter in a pan. Bash up the remaining coriander seeds. Brush a sheet of filo with some melted butter and sprinkle with a pinch of ground cinnamon and a little of the ground coriander. Cover with the second sheet, and brush and sprinkle as before. Repeat for the third sheet. With a sharp knife, cut the stacked filo sheets into two 15cm squares.
Working quickly to prevent the pastry from drying, do the same with the remaining filo sheets. You should now have four squares of filo. Place a generous tablespoon of the sweet potato mixture in the centre of each square and spread out slightly. Brush the edges of each square with water and fold in half, corner to corner, to make a triangle. Press down at the edges to seal. Brush both sides of each triangle with the rest of the melted butter, place on a baking tray and bake in the preheated oven until light brown - this will take about 10 minutes.
Mix the red onion with the watercress, mint and parsley. Mix the yoghurt with the lemon juice, a lug of olive oil and the cumin. Dress the salad with the yoghurt dressing and serve with the hot filo briks and the lemon halves.