In a small bowl, combine mayonnaise, lemon zest, lemon juice, 1 garlic clove, and onion
Season with pepper
Chill and cover
In a large bowl, combine grated onion, rosemary, Worcestershire sauce, 1 garlic clove, 1 tsp. kosher salt and pepper
Gently mix in lamb, shape into 4 four-inch patties, make a slight impression in the center of each patty
Brush onions with oil, season with salt and pepper
Grill burgers and onions on a grill over medium heat for 10 minutes or until internal temperature is 150’, turn halfway through cooking
Lightly grill buns, if desired
Slather buns with mayonnaise mixture
Top with spinach, burger, onion and top with other bun