Spinach Dip Hot Bread

Name: Spinach Dip Hot Bread
Ingredients: ¼ cup plus 2 tablespoons unsalted butter, divided; 2 tablespoons all-purpose flour; ¾ cup whole milk; ¾ cup heavy whipping cream; 1 ¼ teaspoons kosher salt, divided; ¾ teaspoon ground white pepper, divided; 1 ½ cups chopped yellow onion (from 1 medium [10-ounce] onion); 1 (10-ounce) package fresh baby spinach (about 8 cups); 14 ounces pre-shredded Parmesan cheese (about 4 1/4 cups); 1 (24-ounce) sourdough boule; 1 tablespoon chopped jarred Calabrian chiles; 1 ½ ounces pre-shredded low-moisture part-skim mozzarella cheese (about 1/3 cup)