Place onion, chipotles and 1/4 cup water in slow cooker. Stir to combine. Using a knife, make incisions all over pork and insert garlic. Season roast with salt and pepper.
Heat a large pan over medium-high heat and add oil. Put roast in pan and brown on all sides, about 8 minutes. Transfer roast to slow cooker. Pour 1/2 cup water into pan and stir over low heat, using a wooden spoon to scrape up browned bits. Pour liquid into slow cooker. Cover and cook on high for 6 hours.
Remove roast from slow cooker and let cool. Shred pork, using two forks to pull meat apart. Return pulled pork to slow cooker and stir to combine with onions, chipotles and juices.
Serve pork with warm tortillas and toppings of your choice.