Preheat the oven to 180ºC/gas 4. Lightly grease a 20cm x 30cm baking dish with oil.
Get the kids to help you cover the baking dish with a layer of pastry, letting the edges overhang slightly. Brush with a little oil then add another layer of pastry, repeating until you have used it all (there should be 3 or 4 layers).
Boil the kettle, then place the spinach in a colander and pour over the hot water to wilt it (you may have to do this in 2 batches). Push the spinach down with the back of a spoon then when it’s cool enough to handle have the kids squeeze out any excess water, using their hands. Place the spinach on a board and roughly chop, then place in a large bowl and set aside.
Put the grill on high and grill the bacon till crisp, then set aside to cool.
Whisk the eggs in a large bowl with a pinch of sea salt and black pepper. Whisk in the milk and mustard powder, snip in the chives then grate in the lemon zest.
Have the kids crumble the cooled bacon into the spinach and then trim, slice and add the sliced leeks. Mix well and scatter into the filo pastry case. Pour over the egg mixture and carefully place the dish on the bottom of the oven.
Bake for 30 to 40 minutes, or until just set. Leave the pie to cool for 20 minutes before slicing. Delicious served with a crisp green salad.