titus n. owl

Ah, a fajita newbie. Yeah, flank steak is tough, and therefore cheap and considered food for the poor (or was, before fajitas became popular). It's one of those things that rose from humble beginnings to the amazement of the people who originally had to figure out ways to deal with it (another one is mesquite wood, which was once considered a pest, but is now prized for flavoring BBQs).

Anyway, Mark Bittman, in his book How to Cook Everything, says this about grilled marinated flank steak (p. 427):

Flank steak is best for slicing; a marinade gives each slice more flavor....

Flank steak is also good without marinating: just coat it with curry powder, chili powder, or any other commercial or homemade spice rub before cooking.

After cooking, slice the meat against the "grain" -- that is, across its natural striations.

• 4 tbsp lime juice
  
• 2 tbsp soy sauce
  
• 1 tsp minced garlic
  
• 1 tsp ginger (either freshly grated or powder)
  
• 1 tsp sugar
  
• salt and pepper to taste
  
• 1 flank steak, 2 to 2.5 lbs [so adjust the ratios to however much meat you have on hand]

Combine all the seasonings in a bowl and marinate the steak in them for at least 30 minutes (if the marinating time is longer than an hour, refrigerate). Near the end of the marinating time, start a charcoal or wood fire or preheat a gas grill or broiler; make it very hot.

Remove the meat from the marinade and dry well with paper towels. Grill or broil the steak about 4 inches from the heat source for 3 to 4 minutes per side, or until nicely browned. Move the meat to a cooler part of the grill and cook for another 2 minutes per side.

Let rest for 5 minutes before cutting into thin slices.

Disclaimer: I haven't tried this recipe myself, so you're on your own there.