Clean and peel the Vidalia sweet onions. Slice them into rounds approximately ¼ inch thick.
Separate each onion ring into thirds so they cook evenly. Don't leave them nested. (You don't need to completely separate, it's okay if 2 or 3 are attached.)
Spread the onions evenly in the baking dish.
In a small bowl, add the salt, garlic powder, oregano, dry mustard, and cayenne pepper. Whisk together.
Sprinkle the onions with the mixed seasonings.
Cut the butter into eight pats. Add these butter pats evenly across the seasoned onion rings.
Sprinkle both cheeses evenly over the onion rings. Finish with a final topping of shredded Parmesan cheese.
Bake for 35-40 minutes until the cheese is browned and bubbling.
Serve right out of the oven, nice and hot, but still good if you serve warm.
Makes a great potluck dish, or serve on burgers, hot dogs, with meat, on a grilled cheese sandwich, or on pizza