Place the roast in the slow cooker and sprinkle the taco seasoning all over. Add the garlic, onion, and peppers.
In a medium bowl, whisk together the water, lime juice, and tomato paste. Pour the mixture over the beef and vegetables.
Cook the beef:
Cover and cook on high for 4 to 6 hours or on low for 8 to 10 hours. In the last hour, transfer the roast to a cutting board and shred the beef with two forks. Return the shredded beef to the slow cooker, cover, and continue cooking for the last hour.
Serve:
To serve, top each tortilla with some of the beef and vegetable mixture. Add shredded cheese, avocado slices, and pico de gallo.