For the Chicken: 5 tablespoons fresh juice from approximately 3 lemons 4 tablespoons red wine vinegar 4 tablespoons olive oil 2 tablespoons fresh oregano leaves, chopped 2 teaspoons kosher salt 1/2 teaspoon ground black pepper 2 pounds skinless, boneless chicken breast, trimmed of excess fat and cut into 1-inch pieces 4 medium garlic cloves, grated (about 4 teaspoons) For the Tzatziki Sauce: 1 medium cucumber, peeled, seeded, and cut into 1/4-inch cubes (see notes) 1/2 teaspo