Preheat the oven to 350F (180C). Line a large baking sheet with parchment paper.
To make the filling, heat the oil in a skillet, add the pine nuts and pistachios, cook for a minute and add the onions to sweat.
Add the beef and tomato paste, cook until the beef is browned all the way through. Salt and pepper to taste.
Let it cool completely before filling into filo. Add the parsley.
In the meantime, in a small bowl, mix the milk and olive oil.
On the kitchen counter or a table, lay the filo sheets and cover them with a cloth.
Once the filling is cooled, start rolling the pies. Take two filo sheets per pie, brush all over with milk-oil mix. Spread 2-3 tablespoon of filling on one long edge about an inch (2,5 cm) tick. Roll it loosely through the other end then swirl it to give a snail shape.
Place them on the parchment baking sheet.
In a small bowl, beat the egg yolk with a drop of water. Brush it on each pie and sprinkle with nigella or sesame seeds if you are using.
Bake for 35-40 minutes. Let cool for 10 minutes and enjoy!