Preheat your oven to 350°F and position a rack in the center. Lightly grease a 13x9-inch baking dish with cooking spray.

Slice the onions into 1/4-inch thick rounds. Separate the rounds into individual rings and restack them from largest to smallest in the prepared dish to mimic their original shape.

In a small bowl, whisk together the dried parsley, dry mustard, garlic powder, seasoned salt, dried oregano, kosher salt, cayenne, and black pepper. In a separate medium bowl, toss the mozzarella, cheddar, and Parmesan together.

Sprinkle the seasoning mixture over the onions and gently toss to coat. Dot the onions with the butter cubes and pour the vegetable broth into the bottom of the pan.

Cover the dish tightly with aluminum foil and bake for 75 to 90 minutes, or until the centers of the onions are soft and easily pierced with a fork.

Remove the foil and generously sprinkle the cheese mixture over the onions. Return to the oven and bake for 5 to 7 minutes until the cheese is melted and bubbling.

Turn the oven to broil and cook for 1 to 2 minutes until the cheese is browned in spots. Garnish with fresh parsley and serve immediately.