Place onion in a 3 1/2- to 4-quart slow cooker. Trim fat from steak. Sprinkle one side of the steak with cumin, coriander, salt, and black pepper; rub in with your fingers. Cut steak across the grain into thin bite-size strips. Add beef strips to cooker. Top with sweet pepper. Pour broth over all.
Cover and cook on low-heat setting for 7 to 8 hours or on high-heat setting for 3 1/2 to 4 hours.
To serve, use a slotted spoon to spoon beef-vegetable mixture onto tortillas. Top each serving with carrot and lettuce. Fold tortillas over. Serve with salsa, guacamole, and sour cream.