1. Preheat oven to 180C. Fry the onion in a little butter until golden brown and caramelised (about 10 – 15 mins). Fry the leeks in butter until soft. Season to taste.
2. Put the leeks, onions, cheese, spinach and garlic into a mixing bowl and mix lightly.
3. Butter a large baking dish and lay 3 sheets of filo pastry on the bottom, buttering each layer using melted butter and a pastry brush and trimming sheets to fit if needed.
4. Spread 1/3 of the leek mixture over the pastry and add 3 more sheets of filo pastry. Add another 1/3 of the mixture then 3 more sheets of filo, and finally the last of the mixture topped with the last 3 sheets of filo pasty with plenty of butter. Score a criss-cross pattern on the top and bake for around 30 – 40 minutes until crisp and golden.
5. Remove from oven and serve nice big pieces with a simple side salad.
For an extra yummy non-vegetarian version fry some chicken pieces with some chopped garlic and black pepper and add in step 2.