Brush grill rack with oil. Heat grill to medium.
In a large bowl mix together the lamb, 1 1/2 teaspoons cumin, 1 teaspoon coriander, 1/4 teaspoon cinnamon, garlic, salt and pepper. Shape into 4 3/4-inch thick patties. Grill for 5 minutes per side or until done (160°F.)
In a small bowl stir together the ketchup, 1 teaspoon cumin, 1/4 teaspoon coriander, and 1/8 teaspoon cinnamon.
Serve the burgers in buns with lettuce, if desired, and spread with spiced ketchup.