Light your charcoal in the KUDU and let them burn to embers with a layer of white ash.
Loose pack your ground beef to 1-inch patties. Sprinkle with salt and pepper.
Finely chop the onions.
Adjust your cast iron skillet height over the ember at medium heat (approx. 400 degrees).
Move your KUDU cast iron to that height and let it heat up for 5 minutes.
Add a spoonful of duck fat and spread it evenly around the cast iron.
Add your onions and let them sweat and cook down.
Once you start seeing the edges of the onions brown sprinkle in some brown sugar. They will begin to caramelize and char.
Group up your onions into 5 spots in the cast iron roughly the size of your patties and place 5 patties into the skillet on top of the onions.
Let the burgers cook for 5 minutes.
Flip the crusted onions and patties together.
Place the thick cheddar cheese on top of the onions. As it melts add more cheese.
Cook roughly 5 minutes on that side and remove to rest.