CUT steak into 6 equal portions. Rub both sides evenly with pepper.
HEAT 1 tsp. of the oil in large nonstick skillet on medium heat. Add steak; cook 5 min. on each side for medium doneness (160°F). Remove from skillet; cover to keep warm.
ADD remaining 1 tsp. oil to drippings in skillet. Add onions; cook and stir 5 to 7 min. or until crisp-tender. Add steak sauce and sage; cook an additional 2 min., stirring occasionally. Slice steak. Serve relish over the steak slices.