Begin by peeling and thinly slicing the sweet onions, aiming for about a quarter of an inch thickness using a mandolin.

Toss the sliced onions to separate the rings.

In a bowl, combine the salt, black pepper, smoked paprika, granulated garlic, dry mustard, brown sugar, cayenne, and fresh thyme leaves to create a spice blend.

Sprinkle half of this spice mixture over the onions and toss to ensure they are well coated.

Drizzle a couple of tablespoons of olive oil over the onions and mix again.

Add the remaining spice mixture to the onions and toss once more to combine thoroughly.

Grease a 13 by 9-inch baking dish and layer the seasoned onions in the bottom.

Sprinkle two ounces of freshly grated mild cheddar cheese over the first layer of onions.

Next, add two ounces of freshly grated Gruyere cheese on top of the cheddar.

Layer another portion of onions over the cheeses.

On this second layer of onions, add two ounces of Tillamook cheddar cheese.

Top with the remaining two ounces of Gruyere cheese.

Finish by sprinkling about half a cup of freshly grated pecorino Romano cheese on top.

Dot the mixture with three tablespoons of butter, cut into small pieces.

Cover the dish tightly with foil and bake in a preheated oven at 350 degrees Fahrenheit for 45 minutes.

After 45 minutes, remove the foil and continue baking for an additional 30 minutes until the dish is fully cooked.