Preheat the oven to 350°F.

Remove the skins from the onions and slice them into approximately 1/4-inch-thick rings, keeping the rings intact.

Arrange the onion rings evenly in a greased 9x13-inch casserole dish.

Sprinkle the seasonings evenly over the onions.

Cut the butter into small pats and distribute them across the top.

Finish by sprinkling all three cheeses evenly over the dish.

Bake for 30-35 minutes, or until the cheese is melted and golden brown.

Allow the dish to cool slightly before serving.