Bring a large pot of water to a boil; add a big pinch of salt and add the green beans. Cook for about 5 minutes, the beans should still be crisp as they will be cooked more in the pan with the pancetta and mushrooms. Drain them and set aside.
Heat the oil up in a saute pan and add the garlic and pancetta and cook until slightly crispy. Remove the pancetta from the pan and set aside. Add the butter to the fat that has rendered from the pancetta and saute the mushrooms until golden and crispy. Season with salt and pepper. Return the pancetta to the pan and add the blanched beans. Finish with the sour cream and bring to a gentle simmer to reduce and thicken a little and allow the flavours to marry. Give it a final taste and season with salt and pepper, if necessary, before serving.