Heat half of the oil in a large frypan, add the mince and fry until cooked. Drain off excess fat, then place the mince on a plate.
Gently fry the onions, garlic and thyme in the remaining oil in the frypan until soft. Add the meat and tomato puree and sprinkle with the flour. Stir constantly for a few minutes to cook the flour, then add the red wine and Worcestershire sauce. Continue to cook until the liquid has reduced by half.
Add the stock and bring to the boil, then reduce heat and simmer for 30 - 40 minutes. By this time the mixture should be thick and glossy.
While the mixture is simmering, cook the potatoes in salted boiling water until soft and tender. Mash them and then beat in the yolks.
Remove the mince from the heat and allow to cool slightly. Adjust the seasoning if required.
Put the mince mixture into a large deep ovenproof dish and cover with mashed potato. Use a fork to rough up the surface.
Bake for about 30 minutes until bubbling and golden brown.