Season meat generously with salt and pepper. Place meat in slow cooker.
In a food processor or a blender, add the onion, chipotle peppers, lime juice, garlic, cumin and half of the cola. Cover and process until smooth. Add the remaining cola and stir until combined well.
Pour cola mixture over pork.
Cover and cook on high-heat setting for 3 to 4 hours or low-heat for 7 to 8 hours.
Transfer a few pieces of meat to a small bowl and shred it using two forks. Return meat to the slow cooker and repeat process until all of the meat is shredded.
Toast the buns (optional) and spread chipotle mayonnaise on cut sides. Add the shredded pork, and top it with cebollas curtidas and sliced avocado.
FOR THE CHIPOTLE MAYONNAISE
Add all the ingredients to a food processor and puree. Season, to taste, with salt and pepper.
Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used. The calorie count does not include the torta bread or any other topping. However, Mayonnaise has been counted for.