Begin by melting the butter in a pan and softening the onion and garlic in it for 5 minutes.
After that add the chopped bacon and the mushrooms, and cook for a further 10 minutes. Meanwhile, prepare the kidneys: cut them in half and remove the skins, then snip out the cores with a pair of kitchen scissors. Now add the halved kidneys to the pan and cook them to colour a little, stirring them around the pan. Next sprinkle in the flour and stir it to soak up the juices, then add the thyme and pour in the red wine.
Bring the whole lot to simmering point; season with salt (not too much, because of the bacon) and pepper. Then cover the pan and simmer gently for 15 minutes. Then remove the lid and simmer for a further 10 minutes.