For the Secret Sauce:

1. 1. In a small bowl, mix together ketchup, mayonnaise, and Dijon mustard until well combined. Set aside.

For the Patty:

1. 2. Heat a skillet over medium heat and add a drizzle of oil.
2. 3. Add sliced onions to the skillet; caramelize them slowly for about 15-20 minutes until golden brown, stirring occasionally.
3. 4. In a separate bowl, season ground beef with salt and pepper. Form into two equal patties.
4. 5. In another skillet or grill pan over medium-high heat, cook the patties for 4-5 minutes on each side until browned. Top with Swiss cheese during the last minute to melt.
5. 6. Spread butter on one side of each slice of rye bread. On the unbuttered side, layer cheesy patties followed by caramelized onions and a generous dollop of secret sauce; then top with another slice of bread.
6. 7. Grill assembled sandwiches in the same skillet used for onions over medium heat for about 3-4 minutes per side until both sides are golden brown.