Put the spinach in a pan with 2 tbsp water over a low-medium heat and cover. Stir the spinach a couple of times until wilted, about 4 mins, then drain. When cool enough to handle, firmly squeeze the excess water from the spinach with your hands, then roughly chop.
Heat 10g of the butter and all of the oil in a large frying pan. Add the onion and cook over a medium-low heat until pale and softened. Add the drained spinach and garlic, season and cook for about 5 mins, stirring occasionally. Leave to cool.
Transfer the spinach mixture to a bowl, then mix in the spring onions, dates, parsley, mint, pine nuts, feta and egg. Season.
Heat the oven to 190C/170C fan/gas 5. Unroll the filo pastry and cut it into strips 9-10cm wide. Melt the rest of the butter, then brush one strip with some of it before putting another strip on top and brushing that too. Keep the remaining pastry covered with a tea towel as you work. Put two heaped teaspoonfuls of the filling at the end of the strip nearest to you. Fold the bottom corner of the pastry diagonally over the filling, so that what was the bottom edge now meets the right-hand edge of the pastry. Fold over the filled triangle and keep going like this with the whole strip, brushing with butter as you go, until you have a neat triangular filo package. Repeat the process with all the pastry and filling. Brush the top of each one with butter, then put onto baking sheets. Sprinkle with the black sesame seeds and bake for 25-30 mins, or until the pastry is golden brown.