Heat vegetable oil in a large skillet over medium heat until hot. Add onions and sugar; cover and cook, stirring frequently, for about 10 minutes or until onions are golden brown.
Preheat grill.
Combine salt and next 4 ingredients (salt through cumin).
Rub mixture on both sides of flank steak.
Place flank steak on grill and cook for about 4 minutes per side, or until done. Remove from grill and tent with foil. Allow steak to rest for about 5 minutes.
Cut steak against grain, into thin slices. Top each steak with onion mixture and Cilantro Salsa.