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. Download a FREE 1-Week Dinner Plan Shopping List for 5 Healthy Quick-As-Takeout Dinner Recipes!