Melt 2 tablespoons of the butter in a large skillet. Add the shallots and cook over low heat, stirring, until softened, about 5 minutes. Add the mushrooms, season with salt & pepper, cover and cook until they have released their liquid, about 4 minutes. Uncover and cook over medium heat until the mushrooms are browned, about 7 minutes longer. Add the remaining 1 tablespoon butter and cook, stirring, for 1 minute. Season with salt & pepper and transfer to a bowl.