Combine the cumin, smoked paprika, garlic powder, kosher salt, dried oregano, brown sugar, and chili flakes directly in the slow cooker pot. Place the beef roast on top and press the spice mixture evenly into all sides of the meat.
Layer the sliced onion over the roast, then pour the diced tomatoes and beer around and over the onions and beef.
Set the slow cooker to low heat and cook the beef for 8 hours until it is fall-apart tender.
Remove the beef from the cooker and shred it finely using two forks. Return the shredded beef to the slow cooker juices if desired.
Serve the shredded beef hot, garnished with fresh cilantro if you like.
