Boil the potatoes in salted water for 10 minutes. Add the squash, bring back to the boil and cook for another 5 minutes until the potatoes are tender. Drain when ready.
Meanwhile, heat 1 tbsp oil in a wide frying pan, then soften the onions for 10 minutes. Add the curry paste, cook for 1 minute, then add the potatoes, squash and peas, and gently stir. Season well, then cool.
Lightly butter a sheet of filo, then use to line a shallow metal tin, about 27x13cm. Use 3 more sheets, alternating the overhang from the short end and the longest sides of the pan each time.
Spoon in the filling, flatten, then top with the final sheet of buttered filo, folded to fit the top. Fold the overhanging edges back over in reverse order, then mark 8 triangles on the top with a sharp knife.
Brush with the leftover butter. Cover with foil and freeze for up to 3 months.
When ready to eat, preheat in the oven to 200 degrees Celsius, gas mark 6. Bake with the foil on for 50-55 minutes, remove the foil and cook for a further 20 minutes or until piping hot, crisp and golden. Serve with spoonfuls of the tzatziki and the salad.