Whisk together olive oil, lime juice, cilantro, and chipotle chiles in large bowl. Season dressing to taste with salt and pepper. Add Al Pastor, avocado, bell pepper, and onion to dressing. Layer chopped romaine lettuce in tostada crowns or taco shells. Toss to coat. Divide mixture evenly over the tostada crowns or taco shells. Serve and Enjoy.