On a flat top, add oil over medium heat. Add burger patty and cook for 6 minutes; flip halfway. End temperature should be 160ºF. Melt slice of swiss cheese on top of patty.
Lightly butter the rye and toast on flat top, warm caramelized onions on flat top.
Spread the Thousand Island dressing on both pieces of rye, add the cooked patty with cheese and top with onions.