Indian potato pie

INGREDIENTS

• 700g potatoes , sliced
• 400g sweet potatoes , sliced
• 1 onion , chopped
• 1 tbsp olive oil
• 1 tsp cumin seeds
• 2 garlic cloves , crushed
• 1 red chilli , finely chopped
• 1 thumb-size piece ginger , grated
• 1 tsp each ground cumin, coriander and garam masala
• pinch dried chilli flakes
• 200g frozen peas
• juice 1 lemon , plus extra wedges to serve
• small bunch coriander , chopped
• 25g butter , melted
• 275g pack filo pastry
• ½ tsp poppy seeds