Turkey-Mushroom Burgers with Scallion-Lemon Mayonnaise

To prepare burgers: Place bread in a food processor and pulse into fine crumbs. Transfer to a large bowl. Pulse mushrooms in the food processor until finely chopped.

Heat 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add onion, garlic and the mushrooms; cook, stirring occasionally, until tender and liquid has evaporated, about 10 minutes. Add to breadcrumbs and let cool completely, 15 to 20 minutes.

Meanwhile, prepare mayonnaise: Combine sour cream, mayonnaise, scallions, lemon zest, lemon juice, pinch of salt and pepper in a small bowl; whisk until blended.

Preheat grill to medium-high.

Add ground turkey, egg, dill, mustard, 1/2 teaspoon salt and 1/4 teaspoon pepper to the mushroom mixture; mix well with a potato masher. With dampened hands, form the mixture into 6 patties, 1/2 inch thick, using about 1/8 cup for each.

Oil the grill rack (see Tips). Brush the patties with the remaining 1 teaspoon oil. Grill until no longer pink in the center, about 5 minutes per side. (An instant-read thermometer inserted in the center should register 165 degrees F.) Meanwhile, split buns and toast on the grill for 30 to 60 seconds. Serve burgers on buns with the mayonnaise, lettuce and tomato.