This is going to sound weird, but the secret ingredient to Catherine's favorite flank steak is...fish sauce! According to Nathan Myhrvold, marinating the beef in fish sauce will mimic the flavor of dry-aged beef. I don't think it really nails the flavor of dry-aged beef, but I see where he's coming from. It's a richer, more complex and savory flavor.
Use 3g fish sauce per 100g beef and marinate in a ziploc with air squeezed out, for 3 days. (You can do one day if you like, but then I would cut the fish sauce by half). Thoroughly pat dry and then cook like normal. In this case I have sous videnated the steak at 135. If you are cooking sous vide (and now I'm talking to an empty room), do take the marinated steak out of your bag and pat it dry to get rid of excess fish sauce before cooking. (Fish sauce inside the meat = good. Fish sauce outside = fishy).
