Preheat the oven to 350 degrees F (175 degrees C). Spray a 9 x 13-inch baking dish with non-stick cooking spray.
Remove the onion skins and slice the onions into thin slices, about 1/4-inch thick. Separate the sliced rounds into onion rings.
Lay the onions into the prepared baking dish.
Sprinkle the garlic powder, Italian seasonings, salt, and pepper over the onions.
Spread the pats of butter over the onions.
Spread the shredded cheddar cheese over the onions, followed by the Italian blend cheese, then the parmesan cheese.
Bake for 25-30 minutes or until cheese is melted and golden brown.
Serve warm.