Wash the mushrooms to remove any dirt or debris. Slice half of the mushrooms and roughly chop the other half.
Add the olive oil, mushrooms, garlic, and a pinch of salt and pepper to a soup pot. Sauté the mushrooms over medium heat until they have released all of their moisture, all the liquid has evaporated from the bottom of the pot, and the mushrooms become very dark brown.
Add the butter and flour to the pot. Stir and cook for about two minutes more. The flour and butter will form a thick paste and coat the mushrooms and the bottom of the pot. It's okay for the flour to coat the pot and brown a bit, just don't let it burn.
