Preheat the oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.
Spread onion slices evenly in the baking dish. Separate into rings for even cooking.
Drizzle melted butter over the onions. Sprinkle with salt, pepper, garlic powder, and Italian seasoning.
Top with cheddar, mozzarella, and Parmesan cheese, spreading evenly to cover the onions.
Cover the dish with foil and bake for 30 minutes. Remove foil and bake an additional 15–20 minutes, until onions are tender and cheese is golden and bubbly.
Remove from oven, garnish with fresh parsley, and serve warm as a side dish.
Prep Time: 10 minutes | Cook Time: 45 minutes | Total Time: 55 minutes
Kcal: Approx. 250 per serving | Servings: 6