Diner-Style Patty Melt

Heat butter and oil in a Dutch oven. Add onions, season with salt and pepper, cook, stirring continually, for 3 minutes. Cover with a lid and allow onions to caramelize, stirring occasionally to prevent burning, about 20 minutes.

Meanwhile, combine ground beef, garlic salt, and pepper in a bowl and mix thoroughly. Shape ground beef into 4 rectangular patties.

Cook patties in a skillet over medium-high heat, flipping once, 3 to 4 minutes per side. Add a slice of Swiss cheese to each patty and top each with an equal amount of caramelized onions.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Spread 1 teaspoon butter over each slice of bread. Broil until lightly golden and toasted. Assemble patty melts and serve immediately.