Preheat your oven to 200°C, 400°F, gas mark 6. Wash the potatoes and bake in their skins until cooked and soft. Cool, then peel and mash. Grind a 5ml spoon of the coriander seeds and the chillies in a pestle and mortar. Add the cinnamon, stir into the mashed sweet potato and season well. Fold the spring onion into the potato with the egg. Melt the butter gently in a pan. Grind the remaining coriander seeds. Brush 1 sheet of filo with some melted butter and sprinkle with a pinch of 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 into two 15cm squares. Working quickly to prevent the pastry from drying, repeat with the remaining filo sheets. You should now have 4 squares of filo. Place a generous 15ml spoon of potato mix 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 until light brown - about 10 minutes. Mix the red onion with the watercress and herb leaves. Mix the yoghurt with the lemon juice, a glug of live oil and the cumin. Dress the salad with the yoghurt mix and serve with the hot filo briks and the lemon halves.