Savory beef patties, drenched with caramelized onions and melty Swiss cheese, nestled between slices of tangy, buttered rye bread. Imagine a commingling of French onion soup and a juicy burger. It's that good.
Melt 2 tablespoons of the butter in a large skillet over medium-low heat. Add the onion and cook slowly for 20 to 30 minutes, until soft, brown, and caramelized. Don't rush this step; jammy caramelized onions take time!
To make the patties, in a medium bowl, combine the ground beef, Worcestershire sauce, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix well and shape the mixture into 4 equal patties, each about 1 1/2-2-inches thick.
Melt 2 tablespoons of the remaining butter in a separate skillet over medium heat. When the butter is bubbly, add the patties and cook until you reach your desired level of doneness, likely 2 to 4 minutes per side.
To assemble the patty melts, arrange 4 slices of bread on a flat surface. Top with a slice of cheese, hamburger patty, caramelized onions, another slice of cheese, and another slice of bread.
Melt the remaining 2 tablespoons of the butter in a clean skillet over medium heat. Add the patty melts and cook until the bottom is golden brown. Flip and cook until the second side is golden brown and crisp, and the cheese melts.