Slow-cooked onions add moisture and flavor to these lean beef burgers. A quick blend of mayonnaise, ketchup, relish and vinegar makes a perfect tangy, sweet and creamy “special sauce” for this burger. We love the dill relish here, but use sweet relish if you prefer it. Serve with sweet potato fries.
- Preheat grill to medium-high (or see Stovetop Variation).
- Combine onion, oil and 1 tablespoon ketchup in a medium saucepan. Cover and cook over medium-high heat, stirring often, until the onion is softened, 5 to 7 minutes. Reduce heat to medium-low, uncover and cook, stirring occasionally, until very soft, 5 to 8 minutes more. Transfer to a medium bowl and let cool for a few minutes.
- Meanwhile, combine the remaining 1 tablespoon ketchup, mayonnaise, relish and vinegar in a small bowl. Set aside.
- Add beef, Worcestershire (or steak sauce) and pepper to the onion and gently combine without overmixing. Form into 4 patties, about 3/4 inch thick.
- Oil the grill rack (see Tip). Grill the burgers, turning once, until an instant-read thermometer inserted in the center registers 165°F, 4 to 5 minutes per side.
- Assemble the burgers on toasted buns with the ketchup-mayonnaise sauce, tomato slices and lettuce.