Heat vegetable oil over medium heat in a large saucepan; add chopped onion, and cook for 3 minutes or until translucent. Add minced garlic, and cook for another minute. Stir in cumin, cinnamon, salt, and black pepper; cook for 1 minute. Stir in diced tomatoes and green chiles; simmer for 10 minutes. Stir in chopped chocolate and tahini until melted. Stir in cilantro; cool slightly. Process in a blender or food processor for 30 seconds or until smooth. (Add some water if sauce is too thick.) Spoon sauce over a rotisserie chicken. Garnish with sliced avocado and tomatoes; serve.