Ingredients:

  • 2 large yellow onions, thinly sliced (about 400g / 14 oz)
  • 2 tablespoons unsalted butter (30g / 1 oz)
  • 1 tablespoon olive oil (15ml / 0.5 fl oz)
  • 1 teaspoon brown sugar (5g / 0.2 oz)
  • 1/4 teaspoon salt (1.5g / 0.05 oz)
  • 1/4 teaspoon black pepper (1.5g / 0.05 oz)
  • 1 tablespoon balsamic vinegar (15ml / 0.5 fl oz)
  • 1 teaspoon Worcestershire Sauce (5ml / 0.2 fl oz)
  • 1/2 cup mayonnaise (120ml / 4 fl oz)
  • 2 tablespoons ketchup (30ml / 1 fl oz)
  • 1 tablespoon Dijon mustard (15ml / 0.5 fl oz)
  • 1 tablespoon sweet pickle relish (15g / 0.5 oz)
  • 1 teaspoon Worcestershire sauce (5ml / 0.2 fl oz)
  • 1/4 teaspoon garlic powder (1.5g / 0.05 oz)
  • 1/4 teaspoon smoked paprika (1.5g / 0.05 oz)
  • Pinch of cayenne pepper (optional)
  • 1 tablespoon lemon juice (15ml / 0.5 fl oz)
  • 1 pound ground beef (80/20 blend recommended) (450g / 16 oz)
  • 1 teaspoon salt (5g / 0.2 oz)
  • 1/2 teaspoon black pepper (2.5g / 0.1 oz)
  • 4 slices rye bread (approx. 4x4 inch squares)
  • 4 slices Swiss cheese (about 4 oz / 115g)
  • 2 tablespoons unsalted butter, softened (30g / 1 oz)

Instructions:

  1. Sauté onions in butter and oil over medium-low heat. Add sugar, salt, and pepper. Cook, stirring occasionally, until golden brown and softened (approx. 30-40 minutes). Deglaze with balsamic vinegar and Worcestershire sauce. Set aside.
  2. Whisk together all sauce ingredients in a small bowl. Cover and chill in the refrigerator for at least 30 minutes to allow flavors to meld.
  3. Gently mix ground beef with salt and pepper. Avoid overmixing. Form into two equal patties, slightly larger than the bread slices (they will shrink).
  4. Heat skillet over medium-high heat. Sear patties for 3-4 minutes per side, until cooked to your desired doneness (internal temp of 160F/71C for medium). Remove patties from the skillet and let rest for a minute.
  5. Spread softened butter on one side of each slice of bread. Place two slices of bread, butter-side down, in the skillet. Top each slice with a slice of Swiss cheese, one cooked patty, a generous amount of caramelized onions, and a generous dollop of the secret sauce. Top it off with cheese again. Top with the remaining bread slices, butter-side up.
  6. Cook over medium heat for 3-4 minutes per side, or until the bread is golden brown and the cheese is melted and gooey. Use a spatula to gently press down on the sandwiches while cooking. Cut in half and serve immediately.