Instructions:

1. Prepare the Oven: Set your oven to preheat at 350°F. Use cooking spray to lightly grease a 13 x 9-inch baking dish.
2. Mix the Ingredients: In a spacious bowl, take the sliced onions. Season with thyme, parsley flakes, garlic salt, oregano, dry mustard, and cayenne pepper. Ensure a gentle mix to coat the onions evenly with the seasonings.
3. Set the Base: The seasoned onions uniformly in the greased baking dish.
4. Cheesy Layering: Strategically place butter slices over the onions. Followed by generously sprinkling both Cheddar and Gouda cheeses.
5. Initial Baking: Shield the dish using aluminum foil. Place in the oven and let it bake till the onions turn soft and the plate gives a bubbly appearance on the sides. This should take around 40 minutes.
6. Crisping Finish: Take off the foil. Continue baking for another 30 minutes until you achieve a golden-brow