Preheat oven to 180°C fan bake and line a large oven tray with baking paper.
Melt 30g butter in a large frying pan on medium heat. Add leek, garlic and lemon zest. Cook, stirring occasionally, for 5-10 minutes, until leek has softened. Meanwhile, place spinach in a microwave-safe dish and microwave until defrosted (about 5 minutes). Transfer to a colander and press down with a fork to remove excess moisture.
Remove leek mixture from heat and stir in spinach. Leave to cool for a couple of minutes. Stire in eggs, pine nuts, oregano, nutmeg, feta and tasty cheese. Season with a little salt and a generous amount of black pepper
Melt remaining 40g butter. Brush one sheet of pastry with butter, place another sheet on top and brush with butter, then add a third sheet. Add one-fifth of the spinach mixture in a line along with one of the long edges. Roll up into a cylinder and wind the filo into a coil. Place on a prepared tray.
Repeat with remaining pastry and filling, coiling each filo roll around the previous one to form a larger spiral. Brush the pie all over with butter and sprinkle with sesame seeds.
Bake for 45-50 minutes, until pie is cooked through and pastry is golden and crisp.