Preheat oven to 350 degrees Fahrenheit. Grease a 9x13-inch baking dish with non-stick cooking spray. Peel and slice 3 large sweet onions into ¼-inch thick slices. Separate the rings and place them in a large bowl.
Sprinkle with 1 teaspoon garlic powder, 1 teaspoon Italian seasoning, 1 teaspoon kosher salt, ½ teaspoon black pepper, ½ teaspoon paprika, and ¼ teaspoon cayenne pepper. Gently toss to combine.
Place the onions in the prepared baking dish and top with 4 tablespoons butter, cut into small pieces. Layer 1 cup shredded cheddar cheese, 1 cup shredded mozzarella cheese, and ½ cup shredded Parmesan cheese evenly over the onions.
Cover the pan with foil and bake for 30 minutes. Remove the foil and bake for another 30 minutes, until the cheese is golden and bubbly.
Notes
Storing & Reheating
Fridge: Store leftovers in the fridge in an airtight container for 3-4 days.
Reheating: Reheat in the microwave or in the oven at 300ºF until hot.