1. Preheat your oven to 375 F (190 C). Peel the sweet onions and slice them into 1/2-inch thick rounds. Separate the rings and place them in a large mixing bowl.

2. Drizzle the olive oil over the onion rings. Sprinkle the Cajun seasoning, garlic powder, and black pepper evenly over the onions. Toss gently with your hands or tongs until all the onion pieces are well coated.

3. Spread the seasoned onion rings in a single layer in a 9×13-inch baking dish or an oven-safe skillet. Make sure they are not too crowded, as this helps them cook evenly and caramelize slightly.

4. Bake the onions for 15 minutes. This initial baking time softens the onions and allows their natural sweetness to develop before the cheese is added. You’ll notice them becoming tender and slightly translucent.

5. Remove the baking dish from the oven. Sprinkle the shredded mozzarella cheese evenly over the softened onions, followed by the grated Parmesan cheese.

6. Return the dish to the oven and bake for another 5-10 minutes, or until the cheese is melted, bubbly, and lightly golden brown. Keep an eye on it to prevent over-browning.

7. Carefully remove the dish from the oven. If desired, sprinkle with fresh chopped parsley for a pop of color and freshness. Serve hot and enjoy!