Latching on to the first answer, I was able to find what looks like a decent recipe for arrachera, which was previously posted on Yahoo answers:
1½ lbs top grade arrachera (flank steak)
1 cup pineapple juice
½ tbsp salt
2 tsp black pepper
2 tsp garlic salt
2 tsp onion salt
2 tsp hot paprika
1 tsp celery salt
½ tsp ground nutmeg
¼ tsp herbs de Provence
1 large or 2 small white onions, thinly sliced
1 tsp corn or canola oil
2 tbsp Dijon mustard
1 tbsp honey mustard
1 tbsp water
Salt and pepper to taste
At least two hours before serving, remove fat from meat, cut into ¾ inch strips and place in 9 x 13 pan. Put pineapple juice, salt, pepper, garlic and onion salt, paprika, celery salt, nutmeg and herbs de Provence in a small bowl and mix thoroughly. Pour over meat. Stir every 15 minutes.
Heat coals on barbeque grill to very hot. In skillet, stir fry onions about five minutes in mustard, water and salt and pepper. Then cover and simmer over low heat for ten minutes while cooking arrachera. For medium rare, broil the strips about three minutes on each side. Serve the arrachera on heated plates over a bed of onions. Juerg recommends sweet potatoes to finish off this dish.
