Gyro Meat with Tzatziki Sauce

Name: Gyro Meat with Tzatziki Sauce
Ingredients: 1 medium onion, finely chopped or shredded; 2 pounds ground lamb; 1 tablespoon finely minced garlic; 1 tablespoon dried marjoram; 1 tablespoon dried ground rosemary; 2 teaspoons kosher salt; 1/2 teaspoon freshly ground black pepper; Tzatziki Sauce, recipe follows; 16 ounces plain yogurt; 1 medium cucumber, peeled, seeded, and finely chopped; Pinch kosher salt; 4 cloves garlic, finely minced; 1 tablespoon olive oil; 2 teaspoons red wine vinegar; 5 to 6 mint leaves, finely minced