Preheat oven to 210 °C (Gas Mark 7).
Roll out the pastry dough and cut out 8 circles approximately 10cm in diameter with a cookie-cutter or bowl.
Wash the spinach and drain. Peel and finely chop the onion.
Melt butter in a pan and fry onion for 2 to 3 min, then add spinach and allow leaves to sweat. Continue cooking by stirring spinach often for 4 to 5 min. Drain and pour into a bowl. Add salt and pepper. Cut the Chavroux log into 4 slices then cut each of those slices into 2.
Place about a tablespoon of spinach on half of a circle of dough. Add a few pine nuts and a ½ slice of cheese.
Brush the edges of the dough with egg yolk. Then fold half of the dough over the filling to form a turnover. Press down on the edges using the tines of a fork to seal it. Then brush the turnover with egg yolk. Repeat the same process for the other turnovers.
Place on a baking sheet lined with parchment paper. Cook approximately 15 minutes. Turnovers should be golden brown.
Enjoy while warm.
Tips: Frozen spinach can be used instead of fresh if in a hurry. Simply thaw and drain the frozen spinach beforehand.