Brush the caps of each mushroom with a clean sponge. Remove and discard the stems. Slice the small mushrooms thickly and cut the large ones in a large dice.
Heat the olive oil in a large (11 - inch) Dutch oven or saucepan. Add the shallots and cook over low heat for 5 minutes, or until the shallots are translucent. Add the butter, mushrooms, salt, and pepper and cook over medium heat for 8 minutes, until they are tender and begin to release their juices, stirring often. Stir in the garlic and cook for 2 more minutes. Toss in the parsley, sprinkle with salt, and serve warm.