4 Tsp Dill Relish, Coarsely Chopped
3 Tablespoons Parsley Leaves, Coarsely Chopped
1 Tbsp Capers
1 Garlic Clove
1/2 Cup Mayonnaise
1 Tablespoon Whole-Grain Mustard
3/4 Pound Ground Beef, Preferably 80 Percent Lean
1/2 Pound Ground Lamb
Salt And Freshly Ground Black Pepper
4 Thin Slices Of Aged Cheddar
Tomato Slices And Thinly Sliced Cucumber, For Garnish (Optional)
Light a grill. In a mini food processor, pulse the parsley leaves, capers and garlic clove until finely chopped. Add the mayonnaise and mustard and pulse until blended.
In a large bowl, gently mix the ground beef with the ground lamb and season generously with salt and pepper. Form the meat mixture into four 1-inch-thick patties and make a slight indentation in the center of each one with your thumb. Season the burgers with salt and pepper.
Grill the burgers over high heat until they are nicely charred on the bottom, about 4 minutes. Flip the burgers, top them with the cheddar cheese and grill until cooked to medium and the cheese melts, about 4 minutes longer. Transfer the burgers to the rolls and let rest for 5 minutes. Spoon the parsley mixture over the top of the burgers (top with the tomatoes and cucumbers if desired), close the burgers and serve