01 -
Heat butter and olive oil in a large skillet over medium-low heat. Add thinly sliced onions with a pinch of salt and cook gently, stirring occasionally, for 30 to 40 minutes until deeply golden and tender without burning.
02 -
Divide ground beef into eight equal portions and shape each into thin patties approximately 5 to 7 cm in diameter. Season both sides generously with salt and pepper. Press a small indentation in the center of each patty to prevent puffing during cooking.
03 -
Preheat a grill pan or heavy skillet over medium-high heat. Cook patties for 3 to 4 minutes per side until medium doneness. During the last minute, top each patty with a slice of Swiss or cheddar cheese, cover, and allow cheese to melt.
04 -
Slice buns in half and toast cut sides in a dry skillet or under a broiler until golden brown and crisp to prevent sogginess.
05 -
Place bottom bun on plate, layer with one cheesy patty, a generous portion of caramelized onions, the second cheesy patty, more onions, then top with the bun. Garnish with chopped fresh chives and serve immediately.