Yum.
Carne asada is Honduran by descent, and not Mexican like so many people believe. The Mexicans have adopted it, and done wonderful things with it, but at it's root it's definitely a carribean inspired dish. Meat marinated in sour orange juice is a fantastic thing. This is the real deal, like you'd get at a local party in the middle of Honduras.
ingredients
3 pounds of flank or skirt steak
1 large thinly sliced white or yellow onion (slice it across the side, to get rings. more surface area that way)
the juice of 3 oranges (I suppose you could use orange juce too, but it'd have to be unsweetened.)
1/4 cup any oil but extra virgin olive oil, canola recommended
S+P to taste
Marinade the stuff overnight in your fridge. Take it out of the marinade and grill it for about 6 minutes, until nicely browned. Flip it, and do the same again. Carne Asada requires high heat, so either work with hot coals (mesquite coals would rock here) or broil it. Cut it on a diagonal bias and you're good. Brushing it with more of the marinade as it cooks is also not a bad idea. Discard the rest and the onions, because you don't want to eat onions that have been sitting with raw meat. Add ons include a tiny bit of lime juice, cumin, chili powder, and/or chipotle, but really this is a simple dish, so overdoing it with seasoning might be a bit un necessary.
For the taco, simplicity is great. Corn meal tortillas, some chopped onion (red onions are great in this), a bit of chopped cilantro (shredded cabbage is authentic, too). A bit of guacamole isn't exactly authentic, but it is crackalicious. These really don't need cheese, the meat is tender enough as is, but if you do use cheese, use a sharp hard white cheese. It's more authentic that way.
Carne asada can be eaten alone, as well, I just think the taco is a great use for it. You could also serve it over a bit of seasoned long grain rice.