1Peel and slice onions 1/4 inch thick using a mandolin. Toss onions to break up the rings.
2Mix salt, pepper, paprika, granulated garlic, dry mustard, brown sugar, cayenne, and thyme leaves in a bowl.
3Toss the onions with olive oil to coat.
4Sprinkle half of the spice mixture over the onions and toss with hands, then repeat with the remaining half of the spice mixture.
5Arrange a layer of onions in a greased 13x9 baking dish, then add 2 oz of cheddar and 2 oz of Gruyère cheese. Add another layer of onions followed by the remaining 2 oz of cheddar and 2 oz of Gruyère.
6Sprinkle 1/2 cup of grated pecorino romano over the top and dot with small pieces of butter.
7Cover tightly with foil and bake at 350 degrees F for 45 minutes.
8Remove the foil and bake for an additional 30 minutes.
Ingredients
3 lb sweet onion, peeled and sliced 1/4 inch thick