Combine onion-soup mix, salad oil, vinegar, and soy sauce; add lamb cubes and stir to coat.
Refrigerate overnight or let stand at room temperature 2 or 3 hours, turning meat occasionally.
Sprinkle with freshly ground pepper.
Broil 4 to 5 inches from heat 8 to 10 minutes or till done, basting now and then with marinade.
Serve on pilaf, if desired.