Place a large frying pan over a medium heat and add the olive oil. Saute the onions until soft before removing from the heat and adding the spinach, spring onions, lemon juice, sumac, nutmeg, and salt and pepper. Once the spinach has wilted place into a colander to drain and cool slightly. Once cooled remove from the colander and add the pine nuts and toss to combine. Taste and adjust seasonings as needed.
Preheat the oven to 190 degrees celsius (375 Fahrenheit) and cut the 12 filo sheets in half. Take one sheet from the stack and cover the remaining sheets with a clean tea towel to prevent them from drying out.
Brush the filo sheet with the butter and oil mixture and fold in half lengthways. Brush again and place a heaped spoonful of the mixture close to the corner of one of the short sides of the pastry. Fold the corner over to encase the filling, alternating left or right. Follow with a final brush of the butter and oil mixture and place on a baking tray lined with baking paper while you continue with the remaining pastry and mixture.
Bake the triangles for 15 to 20 minutes, or until crispy and golden.