Heat up your pan, add butter, olive oil, sliced onions, and salt & pepper.Cook on low 45 min to an hour (mixing often), until your onions are caramelized.
Cook the Bayonne ham on tin foil on the grill next to the burgers.Top burgers with caramelized onions & sliced brie, let it melt.Add aioli, lettuce, and top it with crispy Bayonne ham.