Tunisian brik

INGREDIENTS

• 2 medium sweet potatoes
• 1½ teaspoons coriander seeds
• 2 dried red chillies
• ½ teaspoon ground cinnamon, plus a little extra
• sea salt
• freshly ground black pepper
• 2 spring onions, trimmed and finely chopped
• 1 large free-range egg, beaten
• 150 g butter
• 6 sheets filo pastry
• 1 red onion, finely sliced
• 1 bunch fresh watercress
• 1 bunch fresh mint, leaves picked
• 1 bunch fresh flat-leaf parsley, leaves picked
• 4 tablespoons fat-free natural yoghurt
• 3 lemons, 1 juiced, 2 cut in half
• extra virgin olive oil
• 1 pinch ground cumin