Let the clams soak 20 minutes in salted water to remove the sand. Brush well and eliminate those that have opened.
If necessary, clean the girolles under running water using a brush. Blot well dry.
Heat the oil in a large cooking pot, on medium heat, add and sauté the pancetta with the onions. Once the onions are translucent, add the girolles. Stir and cook 2-3 minutes. Add the remaining vegetables and butter and stir. Cook 5 minutes. Increase cooking temperature to high heat, add milk and salt. Bring to a light boil. Add the clams, cover and reduce temperature to medium heat and bring the liquid to simmering point. Cook 5-10 minutes or until all the clams have opened.
Pepper generously. Serve with a chive garnish.
A few girolles will also make a nice garnish: sauté in oil until golden and deposit on the soup upon serving.