Trim excess fat from the pork and cut into 4 pieces. Season on all sides generously with salt.
Put the salsa, chipotle chili, adobo sauce, cumin, smoked paprika, garlic and 1 teaspoon salt in a slow cooker and stir to combine.
Add the onion and pork to the slow cooker and turn to coat with the sauce. Cook on low for 6-8 hours or on high for 4 hours.
Transfer the pork to a bowl and shred with two forks. Pour the sauce through a strainer and skim the fat with a spoon or a fat separator if you like. Add enough sauce to the pork to moisten it.
To serve, warm the tortillas and fill with pulled pork and desired garnishes.