Heat the olive oil in a heavy-bottomed pan and add the leeks and onion. Season and cook over a medium heat until the leeks and onion are soft, stirring frequently. Add the garlic and cook for another 2 mins. Make sure that the mixture isn’t too ‘wet’ or it will make your pie soggy. If it is, turn the heat up to cook off the excess moisture. Add the artichoke, then set aside to cool.
Mix together the ricotta, parmesan, egg and herbs together in a large bowl, then tip in the cooled vegetables. Season and mix with a fork.
Clear a big space on your work surface or kitchen table – you’re going to construct a pastry that is one metre wide. Take one piece of filo, put it on the work surface with the long side facing you and brush it with butter. Put two more pieces on top, brushing each one with butter. Now do the same again with three layers, overlapping the previous sheets to the left side by about 3cm. Keep doing this until you have one metre of overlapping buttered pastry sheets.
Heat the oven to 220C/200C fan/gas 7 and put in a baking sheet to warm up. Spoon the filling in a thin layer onto the pastry, about 2.5cm from the edge. Keep going until you have filled the entire length. Carefully roll this up, working from the edge nearest you, until you have a long sausage shape. You need to be firm, but don’t roll too tightly or the filo will split. You should have a couple of unused sheets of filo left, so use these to patch things up, if you need to.
Working from one end of the sausage shape, coil the pastry, then carefully transfer onto a baking sheet (about the same size as the one warming in the oven).
Brush the pie with butter, sprinkle over the sesame seeds, then slide the baking sheet onto the one that’s already in the oven (this helps to ensure even cooking). Bake for 30-35 mins, or until golden. Leave the pie to cool a little before eating.