For the beef

1. Make braising sauce. In a medium bowl, mix garlic, ginger, brown sugar, soy sauce, rice vinegar, and sesame oil together until sugar is dissolved. Set aside.
2. Sear beef. Heat 2 tablespoons of canola oil in a large heavy skillet over high heat. Add tied roast to the pan and sear on all sides. Add more oil as needed.
3. Cook beef. Place roast in a slow cooker and pour soy sauce mixture over the roast. Cover and cook on low heat until meat is very tender, 6 to 8 hours.
4. Make sauce. Transfer the roast to a serving platter and tent with foil to keep warm. Strain the cooking liquid through a fine mesh strainer into a small saucepan. Skim fat off the top of the sauce. Simmer the sauce over medium-high heat until reduced by one quarter to one half, 15-20 minutes.
5. Shred beef. Meanwhile, remove cooking twine from the roast and shred the meat. Pour some of the heated sauce over the meat. Use just enough sauce to moisten the meat. Reserve the remaining sauce for serving.

For the slaw

1. Toss. In a medium bowl, mix together the rice vinegar, sugar, and salt. Add the sliced onions to the mixture and let sit for 5-10 minutes. Add carrot ribbons and cilantro; toss to combine.

For serving

1. Assemble tacos. Make tacos by layering desired amounts of sliced avocados, shredded beef, carrot slaw, and cilantro leaves in warm tortillas. Drizzle with reserved sauce and a squeeze of lime, roll up, and serve.