Trim off any excess bits of fat from the meat and cut into 2-3 inch chunks.
Add the meat to the slow cooker along with: 3 minced chipotles in adobo, 1 tablespoon adobo sauce, 1 chopped onion, 5 minced garlic cloves, 2 teaspoons cumin, 1 tablespoon Mexican oregano, 2 teaspoons salt, freshly cracked pepper, the juice of 2 limes, and a splash of water.
Give everything a quick stir and cook in the slow cooker on low for 6-8 hours or on high for 4 hours. When the beef pieces are fully cooked inside and fork tender they are done.
Use two forks or a spatula to shred the beef.
Let the shredded beef cook in the juices for an additional 10-15 minutes (this is the most important part!). You can also taste for seasoning at this point. I added another generous pinch of salt to this batch.
Once the beef has stewed in its own juices it is ready to be served. For a simple barbacoa taco, serve the beef in a warm corn tortilla with pickled onions and freshly chopped cilantro.
Store leftovers in their juices in an airtight container in the fridge.