Set a large ovenproof pot or Dutch oven over medium-high heat, add a drizzle of oil and brown the beef in batches, sprinkling with salt. As it browns, transfer to a plate—or a slow cooker, if you’d like to use one.
Add the onion to the pan and cook for a few minutes.
If you’re using a slow cooker, add the remaining ingredients to it (using 1/2 cup stock) along with the onion. Otherwise preheat the oven to 300˚F, return the beef to the pot, sprinkle with chilli powder and add the salsa and 1 cup stock, stirring to loosen any browned bits in the bottom of the pan.
Cover and cook for 3 hours, or until the meat is very tender. (In the slow cooker, cook on low for 6 hours.)
Shred it with forks and serve wrapped in tortillas with your choice of additions and condiments.