- 1 cup mayonnaise
- 2 tablespoons finely chopped fresh chives
- 1½ tablespoons finely chopped fresh parsley
- 2 teaspoons finely chopped fresh thyme leaves
- 2 cloves garlic, minced
- 1 teaspoon freshly grated lemon zest
- 1 teaspoon fresh lemon juice
- 1½ pounds ground chuck (80% lean)
- 1 teaspoon kosher salt
- ¼ teaspoon freshly ground black pepper
- 1 large yellow onion, cut crosswise into four ½-inch-thick slices
- 2 tablespoons extra-virgin olive oil, divided
- 5 ounces Fontina cheese, thinly sliced
- 4 Hawaiian sweet sandwich buns, or other soft sandwich buns, split
- 4 large leaves butter lettuce
In a small bowl whisk the mayo ingredients. Cover and refrigerate.
Mix the ground chuck, salt, and pepper, and then gently form four patties of equal size, each about ¾ inch thick. With your thumb or the back of a spoon, make a shallow indentation about 1 inch wide in the center of the patties to prevent them from forming a dome as they cook. Cover and refrigerate the patties until ready to grill.
Prepare the grill for direct cooking over medium-high heat (400° to 500°F).
Lightly brush the onion slices on both sides with oil, and brush the cut side of the buns with oil. Grill the onion slices and the patties over direct medium-high heat, with the lid closed, until the onions are lightly charred on both sides and the patties are cooked to medium doneness (160°F), 8 to 10 minutes, turning once. During the last 30 seconds to 1 minute of grilling time, place the cheese on the patties to melt, and toast the buns, cut side down, over direct heat.
Build each burger on a bun with mayo, a patty, onions, and a lettuce leaf. Serve warm.