Preheat the oven to 180c
Line a large baking tray or roasting tray with greaseproof paper. Toss the butternut squash in a large bowl with the oil and a pinch of salt and pepper. Spread out on the baking tray and put in the oven to roast for 20 minutes until a fork can be inserted easily into the cubed squash. Warning - don’t overcook the squash at this point - it should be slightly al dente. While the squash roasts, prepare the rest of the filling.
In a medium frying pan over medium heat, add a drizzle of olive oil. When hot, add the onion along with the whites of the spring onions. Fry for a minute then add the garlic and cook until the onion is turning translucent. Add the spices and cook for a few more minutes. Remove from the heat and allow to cool completely.
When the squash is cooked, remove from the oven and allow to cool completely. (We’ll be using the oven again to bake the pie at the same temperature but since the filling can take some time to cool, it may be more economical to turn it off at this point). Mix the squash with the onion and spice mixture. Add the onions and chopped coriander and mix again.
Preheat the oven to 180c.
To assemble the pie, line a 20cm cake tin or tart tin with greaseproof paper. Take one sheet of filo and lay it in a diamond figuration so that one corner of the square points upwards towards 12 o clock and hangs over the outer edge of the tin. Brush the filo with a little olive oil and sprinkle over a tsp of panko. Rotate the tin and repeat the process with your next layer of filo, making sure the overlapping corner is pointing a couple of hours further around the clock. Brush with oil, sprinkle with panko and repeat, again rotating until you’ve used all the pastry sheets and the corners of the filo are all pointing out in different directions.
Brush the last layer of filo with some oil and then add the cooled filling to the tin and flatten (but don’t compreswith a spoon. Starting with the top sheet, fold over the squares of filo into the centre of the pie, bunching each sheet slightly to create a roughly folded effect. If the sheets don’t overlap properly and you have a hole of filling peeping through, don’t worry! Just take an extra sheet of filo, bunch it slightly and place it in the centre of the pie to fill the gap.
Brush the top of the pie with oil, sprinkle with nigella seeds and pop in the oven for 30 minutes or until the top is crispy and golden. Remove from the oven and remove the pie from the tin. Place the pie on a baking tray and return it to the oven to bake for another 15 minutes, to ensure the sides and base are crispy too.
Remove from the oven and serve immediately. The filo will become soft if you don’t serve and consume straight away, however ten minutes in the oven will crisp everything back up.