Tennessee Onions

Name: Tennessee Onions
Ingredients: 4 tablespoons unsalted butter, cut into small cubes, plus more for greasing; 4 Vidalia onions (about 2 3/4 pounds); 3/4 teaspoon mustard powder; 3/4 teaspoon dried oregano; 1/2 teaspoon garlic powder; 1/2 teaspoon onion powder; 1/2 teaspoon sweet paprika; Kosher salt and freshly ground black pepper; 3/4 cup shredded mild Cheddar (about 3 ounces); 3/4 cup shredded Gouda (about 3 ounces); 1/2 cup grated Parmesan (about 1 1/3 ounces); Chopped fresh parsley, for garnish