Step 1

Preheat oven to 350ºF (177ºC). Lightly butter a 9X13 inch baking dish or spray with nonstick cooking spray; set aside.

Step 2

In a small bowl, mix together thyme leaves (1 teaspoon), garlic powder (1 teaspoon), cajun seasoning (1 1/2 teaspoons), dry mustard (1/2 teaspoon), and salt (1/4 teaspoon).

Step 3

Add rings of onion (2 1/2 pounds) to a large bowl. Sprinkle with seasoning mix and toss gently to evenly coat.

Step 4

Arrange onions in the prepared baking dish.

Step 5

Top onions with butter slices (8 thin slices), then sprinkle a layer of mozzarella (1 cup) then a layer of cheddar (1 cup) evenly over top.

Step 6

Spray a large piece of aluminum foil with nonstick cooking spray. Cover baking dish with foil, nonstick spray side down on onions. Bake in preheated oven for 45 minutes.

Step 7

Remove foil and discard. Sprinkle parmesan (1/2 cup). Return to oven uncovered and continue to bake until onions are soft and the top is golden brown, about 25-30 minutes.