Heat 2 tbsp of the olive oil in a non stick frying pan and add the breadcrumbs. Fry stirring until golden and crunchy, around 5 minutes. Remove from the pan onto a plate lined with kitchen paper, drain and set aside.
Heat the cumin and mustard seeds in a dry pan until the mustard seeds begin to pop and the spices smell aromatic.
Take off the heat and place in a pestle and mortar with the chilli flakes and pound until you get a coarse powder. Set aside.
Heat 2 tbsp of oil in the frying pan and add the leeks, add a pinch of salt and fry until soft and golden, around 20 minutes.
Put the grated courgettes in a clean tea towel and squeeze out any excess liquid and then add the courgettes to the leeks and cook for another 5 minutes.
Increase the heat slightly and add the garlic and ground spices to the pan and cook for another 2 minutes until softened. Transfer all to a mixing bowl and leave to cool for a few minutes.
Mix in the feta, lemon zest, egg and season with lots of black pepper and a little salt, go easy on the salt as feta is very salty.
Take a 22cm pie dish and spoon the mixture into the dish. Toss the sliced courgette with the remaining 1 tbsp olive oil and then layer them in a spiral pattern over the top fo the filling. Sprinkle with the cheese and bake for 30-35 minutes or until golden brown. Serve immediately scattered with the breadcrumbs and served with a green salad.
Assemble and bake the day before, cool, cover and keep in the fridge. Cook in a preheated oven from chilled until piping hot all the way through.