Heat the oven to 350ºF. Lightly coat a 9x13-inch baking dish with cooking spray. Separate 3 pounds sliced Vidalia onions into rings and place in a large bowl. Add 1 ½ teaspoons kosher salt, 1 teaspoon dry mustard powder, 1 teaspoon garlic powder, 1 teaspoon smoked paprika, and ¼ teaspoon cayenne pepper, and toss to coat.

Arrange half of the onion rings in an even layer in the baking dish. Sprinkle evenly with 2 ounces shredded mild cheddar cheese and 2 ounces shredded smoked Gouda cheese (about ½ cup each).

Arrange the remaining onions on top and sprinkle with the remaining 2 ounces each shredded mild cheddar and smoked Gouda. Sprinkle evenly with 1 ¼ ounces grated Parmesan cheese. Scatter 4 tablespoons diced unsalted butter on top.

Cover the baking dish with aluminum foil. Bake until the onions are softened and the casserole is bubbling, about 45 minutes. Uncover and bake until the cheese on top is melted and browned in spots, about 30 minutes more. Sprinkle with 2 tablespoons finely chopped fresh parsley leaves if desired.