Cube roast into large 2-inch chunks. Try to cut off any larger pieces of fat. Season beef well with salt and pepper.
In a large skillet, add a good drizzle of oil over medium-high heat. Add chuck roast cubes and sear well on all sides until browned. Transfer browned beef pieces immediately to a slow cooker.
When all beef is browned, add onions, ginger, and garlic to the skillet along with some water to scrape up any bits stuck to the pan. Cook for a minute and then transfer liquid and veggies to the slow cooker.
Add all other beef ingredients to the slow cooker along with one cup of water. Stir together, cover, and let cook for 6 hours on low.
For slaw, grate carrots and shred cabbage. Remove seeds from chiles and mince finely. Stir together with a big pinch of kosher salt, sugar, and rice wine vinegar. Store in the fridge until needed.
When beef is done, remove and shred. Pour out liquid from slow cooker, but save it. Add beef back to the slow cooker along with enough liquid to make it moist, but not super-wet. Keep warm until serving.
To make a taco, heat a flour tortilla in a skillet or in the oven. Top with shredded beef, cabbage slaw, fresh cilantro, and sesame seeds.