Recipe: Creamy Cabbage Gratin with Bacon and Mushrooms

Name: Recipe: Creamy Cabbage Gratin with Bacon and Mushrooms
Ingredients: Cooking spray or butter; 6 ounces bacon, cut crosswise into thin strips; 8 ounces fresh wild mushrooms or cremini mushrooms, chopped if large; 1 medium yellow onion, thinly sliced; 1 large cabbage (about 3 pounds), cored and coarsely chopped, or 16 cups shredded cabbage; 1/4 cup all-purpose flour; 2 cups whole milk, warmed; 2 cups grated Gruyère cheese (about 4 ounces), divided; 1 teaspoon kosher salt, plus more for seasoning; 1 teaspoon freshly ground black pepper; 1 teaspoon dry mustard; 1/2 teaspoon freshly grated nutmeg; 1/2 teaspoon cayenne pepper, or to taste; 2 ounces walnut pieces (1/2 cup); Chopped fresh parsley leaves, for garnish