Place butter and oil into a saucepan or frying pan and melt over medium heat. Once butter is foamy, add the leek, onion and a good pinch of salt. Cook, stirring often, for 5-6 minutes or until softened and sweet, without colouring.
Add the garlic to the onion mixture and cook for a further 2 mins. Turn off the heat and add the spinach. Stir until spinach has wilted. Transfer mixture to a bowl and allow to cool to room temperature.
Add the herbs, chicken, feta, parmesan, nutmeg, salt, pepper and 1 lightly beaten egg. Stir until well combined. If not making the triangles immediately, cover and chill until required.
Preheat oven to 200℃ (180℃ fan forced). Line 2 large baking trays with baking paper. To make 16 small triangles, cut each sheet of pastry into 4 equal squares. Place a heaped tablespoon of the filling mixture onto one side of the pastry. Carefully fold the pastry over to enclose in a triangle shape (or make a rectangle, up to you). Press the edges to seal, then crimp with a fork. Transfer to a prepared baking trays. Repeat with remaining pastry and filling.
OR to make 8 large, cut each sheet into 2 equal triangles, for a triangle shape or in half down the middle to give you 2 equal rectangles if you wish to make square pastries. Divide the mixture evenly between the 8 sheets of pasty onto one half. Fold the pastry over, press to seal, crimp the edges then slice 2-3 slits in the pastry tops. Brush with beaten egg yolk (or whole egg) and sprinkle with sesame seeds, if using.
Using a pastry brush, brush the egg yolk (or whole lightly beaten egg) over the tops of the pastry triangles to cover evenly. Sprinkle with sesame seeds. Bake in the oven for 35-40 minutes or until puffed and golden brown. Serve hot, warm or cold. If desired, serve with a relish or chutney on the side.