Preheat the oven to 150°C (300°F/Gas 2).
Season the lamb with salt and pepper and toss with the flour until evenly coated. Heat the oil in a large ovenproof heavy-based pan over high heat.
Add the lamb in batches and cook for 1–2 minutes, or until well browned, then remove. Add the onion with a pinch of salt and cook over low heat for 5 minutes, or until softened.
Add the tomato paste and extra flour and cook for a minute or so. Add the red wine and veal stock and stir until the mixture boils. Return the lamb to the pan, cover the pan with foil, place in the oven and cook for 1 hour, or until the lamb is tender.
Stir through the peas and mint and adjust the seasoning if necessary.
Increase the oven temperature to 200°C (400°F/Gas 6). Divide the pie filling among four 300 ml (101/2 fl oz) ramekins or pie dishes. Top each one with a piece of pastry large enough to hang over the edge of the dish (it is important to remove the pastry from the freezer only 5–10 minutes before you need it, so it thaws but stays chilled).
Press the pastry down firmly around the edges of the dishes, and brush evenly with the egg yolk. Bake in the middle of the oven for about 15 minutes, or until puffed and golden. Serve warm.