Yield six to eight servings Ingredients 8 tablespoons (1 stick) unsalted butter 1 pound button mushrooms, thinly sliced 1 cup finely chopped yellow onion 3 tablespoons minced garlic clove 1/2 cup all-purpose flour 7 cups milk 2 teaspoons kosher salt 1/2 teaspoon freshly ground pepper 1/4 teaspoon