Creamy Cabbage Casserole

Name: Creamy Cabbage Casserole
Ingredients: 3 tablespoons unsalted butter, divided; 11 cups sliced green cabbage (from 1 medium head); 1 large sweet onion, thinly sliced (about 3 cups); 2 large celery stalks, thinly sliced crosswise; 16 whole-grain round buttery crackers (such as Ritz), crushed; ¼ cup all-purpose flour; 2 cups reduced-fat milk; 1 ½ teaspoons Dijon mustard; 1 teaspoon salt; ½ teaspoon cayenne pepper; ¼ teaspoon ground nutmeg; 1 cup finely shredded Monterey Jack cheese, divided