For a richer sauce, consider adding a tablespoon of balsamic vinegar during the simmering stage. If you don't have fresh basil, dried basil can be used, but use about half the amount as the flavor is more concentrated. The strawberry salsa is best when made a few hours in advance to allow the flavors to meld. Be sure to store it in the refrigerator. To ensure even cooking, bring the flank steak to room temperature for about 30 minutes before grilling.