Adjust oven rack to middle position. Heat oven to 300 degrees.
In a small bowl, mix sour cream, yogurt, cucumber, vinegar, 1 minced garlic clove, and salt and pepper to taste; set aside.
Place pitas in oven; bake until warm and pliable, 7 minutes. Cut in half. Meanwhile, heat oil in a large skillet over high heat. Carefully add onion; saute until spotty brown but still crisp, 2 to 3 minutes. Add turkey, oregano and remaining minced garlic; continue to saute until heated through, another 2 minutes.
Serve, letting guests fill their own pitas with lettuce first, followed by turkey, tomatoes and cucumber sauce.
