Cut the mutton into small 1.5 cm cubes and finely dice the onion.
Make the pastry by rubbing together the flour, butter, lard and salt until breadcrumb texture forms.
Add cold water gradually, mixing until the dough just comes together. Wrap in clingfilm and refrigerate for 30 minutes.
Heat the oven to 180°C. Place the mutton cubes in a heavy based pan and brown on all sides over medium high heat for 5 minutes.
Add the diced onion and cook for 2 minutes until softened.
Pour in the stock, add thyme and black pepper, then simmer for 15 minutes. The meat should be tender but not fully cooked. Remove from heat and cool completely.
Roll out two thirds of the pastry to approximately 3 mm thickness and cut out 4 circles of 10 cm diameter for the pie bases.
Place each pastry circle on a baking tray. Divide the cooled meat filling equally amongst them, spooning it onto the centre of each base.
Roll out remaining pastry and cut 4 circles of 10 cm diameter for the lids. Brush the edges of the bases with water, then place the lids on top and press edges firmly to seal.
Prick the top of each pie with a fork. Mix the egg yolk with 1 tablespoon water and brush over the tops.
Bake for 25 to 30 minutes until the pastry is golden brown and crisp.
Cool for 5 minutes before serving.
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