Place a large, deep pot on the stove, pour in enough olive oil just to cover the bottom and heat until the oil is hot.
Add the mince in batches and fry until well coloured, adding more oil to the pan if needed. Turn down the heat to medium, sprinkle with the flour and then add the onion, carrots, celery and mushrooms and sauté for about 10 minutes.
Pour in the stock and add the thyme and a good grind of Salt and pepper. Bring to the boil, remove from the heat and pour the contents into an oven proof dish.
Cover the dish with a lid or foil and cook in the oven for 1 hour, then uncover and cook for a further half hour.
Remove the dish from the oven and cover with the puff pastry; using any leftovers to decorate the top. Return to the oven and cook for a further 30 minutes or until golden brown.
Serve with baby spinach, green vegetables or even a light salad.