Patty Melt 5 Steps
To prepare chili mayo, stir all ingredients in a medium bowl. Refrigerate until ready to use. Mayo will keep well, covered and refrigerated, for up to a week.
To prepare onions, heat oil in a large frying pan over medium heat. Cook onions, stirring occasionally, for 25 to 30 minutes until caramelized. Let cool. If making ahead, onions will keep in the fridge for 4 days.
Divide ground beef into 6 patties. Season both sides with salt and pepper. Set aside.
Preheat oven to 225°F. Spread butter on one side of each bread slice. Sandwich cheese between bread slices with butter sides out. Set a heavy-bottomed or cast iron pan over medium heat. Add sandwiches (you will need to cook in batches). Cook for 3 to 5 minutes per side until bread turns golden and cheese melts. Place on a baking sheet and keep warm in oven until burgers are ready.
Increase heat to medium-high. Coat cast iron pan with oil. Add patties. Cook for 3 to 5 minutes per side until golden and medium-rare. Let burgers rest. Open grilled cheese sandwiches and slot in patties. Top patties with caramelized onions and drizzle of chili mayo.