Chop the onion. Mix olive oil, soy sauce, lemon juice, and cumin. Add the chopped onion and crushed garlic. Add the flank steak and leave to marinate for at least 4 hours, turning the steak half way through.
Preheat the broiler. Scrape the onion off the steak. Strain the marinade and brush it on the meat; grind on a little salt. Broil under high heat until steak is rare (internal temperature of 125 degrees F.), without turning.
Season to taste with freshly ground black pepper. Let stand 5 minutes.