Start with the potatoes. We are not going to boil them as this will remove some of the flavours. Instead, we are going to microwave them until soft. This ensures they retain maximum flavour when mashed. Wrap in a couple of tea towels and microwave on full power for 5 minutes at a time. Keep doing this and perhaps adjusting the time until they are soft to the touch. Brown the mince, drain and remove and set aside. Fry the onion, and carrots in the same pan as the mince adding olive oil if required. Do this until soft and golden. Return mince to the carrot and onion and stir well. Add the flour, mix well and cook for about a minute. Gradually add stock and combine well. Add Worcestershire sauce, tomato puree, stir well and bring to a simmer. Lower heat and cook for about 20 minutes. Add extra water if it gets too dry. Allow cooling slightly then adjust seasoning. Put in a suitable dish and set aside to cool. Remove the skin from the potatoes (these can be baked until crisp for a healthy snack). Pass through a ricer or Mouli, season, add milk and butter to get to your desired consistency. Cover mince with potato, fork a pattern in the top and dot with butter. Cook for 20-25 minutes in 170c until nicely browned.