Rosemary Lamb Burgers

In a small bowl, combine mayonnaise, lemon zest and juice, the small garlic clove, and the chives. Season with pepper. Chill, covered, up to 24 hours.

In a large bowl, combine grated onion, rosemary, Worcestershire sauce, the other garlic clove, 1 teaspoon kosher salt and 1/2 teaspoon pepper. Gently mix in lamb. Shape into four 4-inch patties. Make a slight impression in the center of each patty.

Brush onions with oil, and season with salt and pepper. Grill burgers and onions on the rack of a covered grill directly over medium heat for 10 minutes or until internal temperature is 150° (medium), turning once halfway through grilling time.

Lightly grill buns, if desired. Slather bun tops and bottoms with mayonnaise mixture. Top bun bottoms with spinach, burgers, onion and bun tops.