Heat the oil in a large pan, add the onion and fry until soft, add the garlic for 30 seconds more. Add the spinach leaves, then cover the pan and cook for a few minutes until the leaves are wilted. Stir in the herbs.
Tip the mixture into a large sieve and press out as much liquid as you can. Then return the mixture to the pan and stir in 75g Parmigiano Reggiano, nutmeg and salt and pepper to taste. Leave to cool.
Preheat the oven to 190° C/Fan 170° C/ Gas Mark 5. Unfold the filo pastry. Lay a sheet on the work surface and brush with butter; sprinkle with Parmigiano Reggiano and top with a second sheet of filo. Brush with butter.
Use a pizza cutter to cut the pastry into strips about 5 ½ cm wide. Place a heaped teaspoon of spinach mixture at one end of the pastry, then fold it over to make a triangle. Keep folding to the end of the strip to make a neat parcel.
Repeat this method until the spinach mixture is used.
Brush the filo with any remaining butter, sprinkle with the remaining Parmigiano Reggiano and bake for 20-25 minutes, rotating the trays half way through cooking time, until all are golden brown and crisp. Cool for 10 minutes or so, before serving warm.
Cooking tip: To freeze, place the unbaked parcels on a baking tray and freeze for up to one month, thaw before baking.