01 -
Preheat the oven to 375°F (190°C). Lightly coat a large baking dish with butter.
02 -
Place the onion rounds in a single even layer in the prepared baking dish.
03 -
Sprinkle the onions with salt, black pepper, and Italian seasoning. Distribute butter pats evenly over the top.
04 -
Evenly sprinkle mozzarella cheese over the onions, then layer provolone slices on top. Finish with a generous dusting of Parmesan cheese.
05 -
Place the baking dish in the center rack and bake uncovered for 25 to 30 minutes until the cheese is melted, golden, and bubbling.
06 -
For a crispier top, broil for 2 minutes until the surface develops a golden crust.