Preheat broiler with oven rack 4 inches from heat. Arrange tortillas in a single layer on a baking sheet. Broil 5 minutes or until lightly browned, turning after 3 minutes.
Pulse sour cream, lime juice, salt, 1 avocado half, and 1 Tbsp. cilantro in a food processor or blender 20 to 30 seconds or until smooth, stopping to scrape down sides as needed. Chop remaining avocado half.
Divide chicken among tortillas; sprinkle with cheese. Broil 1 to 2 minutes or until cheese is melted.
Top each with avocado-sour cream mixture, chopped avocado, tomatoes, and desired toppings. Sprinkle with remaining 1 Tbsp. cilantro.