Prepare the mushrooms
Meanwhile, in a medium skillet, melt the remaining 2 tablespoons of butter. Add the shallots, garlic and thyme and cook over moderate heat, stirring occasionally, until the shallots are softened, about 5 minutes. Add the chopped mushroom stems and cook, stirring occasionally, until tender, about 8 minutes. Add the kale and cook, stirring, until tender, about 8 minutes. Add the cream and bring to a boil, then simmer until slightly thickened, about 2 minutes. Remove from the heat and stir in the vinegar and nutmeg. Let the filling cool slightly, then stir in the grated cheese. Season with salt and pepper.