Peel and slice 3 pounds of sweet onions into uniform quarter-inch thick slices using a mandolin
Toss onion slices together to break up the rings
Mix together 2 teaspoons salt, 1 teaspoon black pepper, 1 teaspoon smoked paprika, 1 teaspoon granulated garlic, 1 teaspoon dry mustard, 2 teaspoons brown sugar, ½ teaspoon cayenne, and 1 teaspoon fresh thyme leaves
Coat onions with 2 tablespoons olive oil
Sprinkle half of the spice mixture on onions and toss to coat
Add remaining spice mixture and toss again
Spread first layer of onions in a greased 13x9 baking dish
Add 2 ounces freshly grated cheddar cheese and 2 ounces freshly grated Gruyere cheese
Add second layer of onions on top
Add another 2 ounces cheddar cheese and 2 ounces Gruyere cheese
Sprinkle ½ cup freshly grated Pecorino Romano cheese on top
Dot with 3 tablespoons butter cut into small pieces
Cover tightly with foil and bake at 350°F for 45 minutes
Remove foil and bake for another 30 minutes until golden and bubbly
