In a skillet, heat cooking oil over medium heat. I used ghee/clarified butter to add an extra buttery flavor.
Saute onion for 2 minutes, while stirring. Add garlic and cook for another 2 minutes.
Add mushrooms and cook 2-3 minutes, while stirring.
Add leek and season with thyme, salt and black pepper to taste. Stir well. Cover and cook until leek has softened. This will take about 5-8 minutes. Add a little more butter if needed. Stir occasionally.
Last 2 minutes add spinach. Remove from heat and allow to cool. Add half of the cheese.
Preheat oven to 400F or 200C.
Make discs from dough. Divide leek-mushroom/chicken mixture over the discs. Fold and top with remaining cheese.
Brush the pastry with egg yolk and bake in the oven for 25-30 min.
Serve with green salad and cherry tomatoes on the side.