1. Heat your oven to 160C. Toss the lamb with the flour until well coated. Heat the olive oil in a large dutch oven on the stovetop and brown the lamb in batches,removing each batch from the pan when browned. Add the onion and garlic to the remaining oil in the dutch oven and fry until fragrant and lightly browned. Add the thyme leaves,tomatoes,white wine and stock,scraping up any browned bits sticking to the bottom of the pot.
2. Return the lamb to the pot and cover. Place in the oven for two hours until the lamb is very tender and the stew is very thick. Check the oven 30 minutes before the end of cooking and if the stew is not thick you may need to uncover the lamb for the final part of cooking. Stir through the frozen peas while the stew is still hot and allow to cool to room temperature.
3.To make the shortcrust pastry,combine the flour,salt and butter in a food processor and process to fine breadcrumbs. Add the egg yolk and process until a dough forms. Remove from the food processor and gather into a ball. Wrap in plastic wrap and refrigerate for 30 minutes.
4. Heat the oven to 200C. Roll the dough to a ½ centimetre sheet and line a greased 20-centimetre diameter round deep pie tin or springform pan,leaving one centimetre overhanging the edge of the tin. Fill the pie case with the cooled lamb and pea filling and fold in the overhanging edges of the pie.
5. Brush one piece of puff pastry with beaten egg and place the other piece on top,pressing them together. Cut a circle the size of the pie tin out of the pastry. Brush the overhanging edges of the shortcrust pastry with more egg and place the puff pastry circle on top,pressing to join the two pastries together. Brush the top of the pie with more egg and cut a cross into the top of the pie to let steam escape.
6. Bake the pie for 30-40 minutes until the pastry is golden. Allow to cool slightly and then remove from the tin,slice and serve.