Wash greens; remove coarse stems. Bring greens and water to a boil in a large Dutch oven. Cover, reduce heat, and simmer 1 hour and 45 minutes or until tender, stirring occasionally. Drain well. Return greens to pot.
Whisk together flour, next 5 ingredients, 1 teaspoon salt, and pepper. Add to greens in pot. Add cheeses; pour into a buttered 13" x 9" baking dish.
Combine breadcrumbs, 2 tablespoons melted butter, and remaining 1/4 teaspoon salt, tossing until crumbs are coated. Sprinkle over greens.
Bake, uncovered, at 350º for 1 hour and 15 minutes or until golden. Let stand 15 minutes before serving.
Note: To make 3 cups sourdough breadcrumbs, we used 1/3 (10-ounce) round loaf sourdough bread, torn into pieces, and pulsed in a food processor.
Make Ahead: Cook greens, shred cheese, assemble casserole, and store in refrigerator, without breadcrumb topping, up to a day ahead. Prepare breadcrumb topping, and add just before baking. Bake casserole according to recipe up to 2 hours before serving. Reheat briefly before serving.