Makes 4 burgers

Olive oil (for brushing)

1 cup mayonnaise
1¼ cup chopped fresh basil plus 8 large basil leaves for burger
3 tablespoons chopped drained cornichons or dill pickle plus
1½ teaspoons brine from pickle jar
2 green onions, finely chopped

1¼ pounds ground chicken
4 ½-inch-thick slices country white bread

4 1/3-inch-thick slices large red or orange heirloom tomato

Brush grill with oil; prepare barbecue (medium heat). Mix mayonnaise, 1 cup chopped basil, cornichons, brine, and onions in small bowl. Season basil mayonnaise with salt and pepper to taste.

Mix chicken and ½ cup basil mayonnaise in large bowl. Using wet hands, shape chicken mixture into four ½-inch-thick patties. Sprinkle with salt and pepper. Brush bread with oil.

Grill bread until crisp and golden, about 1 to 2 minutes per side. Transfer bread to plates. Grill chicken burgers until firm to touch and cook through, about 5 minutes per side.

Spread toasted bread slices thickly with basil mayonnaise. Top each bread slice with tomato slice, 2 basil leaves, burger, and dollop of basil mayonnaise. Garnish with remaining basil.