Heat a large saucepan and add two tablespoons olive oil. Add the mushrooms, bacon and onion and fry, stirring, until the onion is soft – about 5 minutes. Add the garlic for the final minute of frying. Spoon out the vegetables and add another tablespoon of olive oil and one tablespoon of butter to the same pan. Brown the kidneys in two batches – don’t overcook them, four minutes does the trick. Remove kidneys and cover to keep warm.
Add the vegetables back to the pan and add wine to deglaze the pan. Boil for a minute, then add the water, thyme and bay. Cover and simmer for 10 minutes. Taste and add salt as needed (it will need a good bit) plus ten twists of black pepper. Knead the remaining tablespoon of butter with the flour to form a roux. Divide into three or four lumps and add them to the sauce, one at time, stirring all the while. Simmer the sauce for a few minutes until the flour starts thickening the sauce. Return the kidneys to the pan along with any resting juices and heat through.
Remove the bay leaf and thyme stalk, sprinkle with chopped parsley and serve on mash, polenta, toasted sourdough or ciabatta, or with ribbon pasta.