In a small bowl, mix together the chili powder, cumin, paprika, sea salt, garlic powder, onion powder, red chilli flakes, oregano, and black pepper.
Rub the spice mixture all over the beef, ensuring it is well coated.
Place the beef in the bowl of a slow cooker and pour in ¼ cup of water.
Cover and cook on high for 6 hours or low for 8–10 hours, until the beef is tender and easily shreds.
Once cooked, use two forks to shred the beef directly in the slow cooker. Mix the shredded beef with the remaining cooking liquid.
In a bowl, mash the avocados with the juice of 1 lime. Season with garlic powder and salt to taste.
Heat the corn tortillas in a frying pan over medium heat for about 1 minute on each side, or directly over an open gas flame for a few seconds until slightly charred and warm.
To assemble, spoon some of the mashed avocado onto each tortilla. Top with a generous portion of shredded beef.
Drizzle with your choice of dairy-free yoghurt, sour cream, or mayonnaise. Serve with extra lime wedges and fresh coriander leaves.