Rub cumin and pepper over Chuck roast. Heat oil in a large skillet over medium-high heat. Cook roast for about 3 minutes on each side.
Place roast in the slow cooker. Add salsa verde, onion and garlic. Cover and cook on LOW for 8 hours.
Remove meat from slow cooker and shred meat, discarding excess fat. Return to slow cooker and assemble tacos. Serve with limes.