Preheat your oven to 375°F (190°C).
Spread the sliced onions evenly in a greased baking dish.
In a bowl, mix together mozzarella, Parmesan, cheddar, mayonnaise, garlic powder, Italian seasoning, salt, and pepper.
Spread the cheese mixture evenly over the onions.
Bake uncovered for 25-30 minutes, or until the cheese is golden and bubbly.
Garnish with fresh parsley before serving, if desired.