In shallow nonmetallic dish large enough to hold chicken breasts in single layer, combine lime juice, oil, garlic, cumin and hot pepper flakes.
Add chicken, turning to coat with marinade.
Refrigerate, covered, at least 2 hours or up to 24 hours, turning chicken occasionally.
Peel red onion; cut into 8 wedges.
Skewer each wedge with a toothpick to prevent layers separating; set aside.
Stack tortillas in 2 stacks of 6.
Wrap each stack in foil; set aside.
Remove chicken from marinade, shaking off excess.
Place chicken breasts on oiled barbecue grill; grill over medium high heat 8 to 10 minutes, turning once, until golden brown and no longer pink inside.
Keep warm.
Place wrapped tortillas on barbecue grill; heat 3 to 4 minutes, turning once.
Place onion wedges on barbecue grill; grill 3 to 4 minutes, turning once, until browned and tender.
Cut chicken breasts into strips.
Remove toothpicks from onion wedges; separate wedges into layers.
Arrange chicken and onion on heated serving platter; unwrap tortillas.