In a small bowl, combine the mayonnaise, lemon juice, garlic, oregano and salt. Cover and refrigerate until serving.
In a large bowl, combine the spinach, cheese, lemon juice, pine nuts, garlic, oregano, salt and pepper. Crumble beef over mixture and mix well. Shape into four patties.
Using long-handled tongs, moisten a paper towel with cooking oil and lightly coat the grill rack. Grill burgers, covered, over medium heat or broil 4 in. from the heat for 5-7 minutes on each side or until a thermometer reads 160° and juices run clear.
Toast buns on for 1 minute or until lightly browned. Serve burgers on buns with reserved sauce. Yield: 4 servings.