01 -
Heat the oven to 350°F. Butter a 9x13-inch baking dish.
02 -
Peel the onions and slice them into 1/4-inch rings. Separate the rings gently and spread them evenly in the prepared dish.
03 -
Sprinkle the onions with salt, pepper, garlic powder, paprika, and Italian seasoning. Dot the butter slices evenly over the top.
04 -
Cover the dish tightly with foil and bake for 40 minutes, until the onions have softened and released their juices.
05 -
Remove the foil. Sprinkle cheddar, mozzarella, and Parmesan evenly over the onions. Increase the oven temperature to 425°F.
06 -
Bake uncovered for 18-22 minutes, until the cheese is melted, bubbling, and browned in spots. Rest for 10 minutes, then finish with parsley and serve hot.