Heat the oil in a large frying pan until hot; add the mince, season with salt and pepper, and cook until browned. Drain off any excess fat.
In a second large pan, fry the onions, garlic and thyme until soft and golden. Add the browned mince and tomato purée, and sprinkle in the flour. Stir constantly for a few minutes to cook the flour, then add the red wine and Worcestershire sauce and continue to cook until the liquid has reduced by half. Add the stock and bring to the boil, then turn the heat down and simmer for 30 to 40 mins. 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, then beat in the yolks.
Remove the mince from the heat and allow to cool slightly. Make sure it is nicely seasoned, so don't be afraid to try it.
Place the mince mixture into a large baking dish and pipe or fork the mash over the top. Bake in the oven for about 30 minutes until bubbling and golden brown.