Put the potatoes in a large saucepan of cold, salted water, then bring to the boil. Turn down and simmer for 5 mins, add the sweet potatoes and continue to cook for 8 mins until just tender. Drain really
Fry the onion in the oil until soft, add the cumin seeds for 1 min, then stir in the garlic, chilli and ginger with the remaining spices. Cook for a further 2-3 mins, then turn off the heat and stir into the potatoes with the peas, lemon juice, and coriander.
Heat oven to 190C/170C fan/gas 5. Halve the filo sheets, and use two-thirds of them, overlapping, to line a 22cm loose-bottomed cake tin with a little overhang.
As you lay in each sheet, brush with melted butter and keep the rest covered with a clean tea towel. Spoon in filling and press down lightly. Cover with remaining filo, then fold up overhanging sides and scrunch up pastry near the edges.
Poke several slits in the top of the pastry and brush with more butter. Sprinkle with the poppy seeds. Bake for 40-45 mins until golden brown. Serve either hot or at room temperature with lemon wedges.