Combine all the ingredients but the meat in a large bowl or zip lock bag. When mixed well, marinate for at least 4 hours but ideally, overnight or 24 hours.
When ready to cook, get your grill or grill pan nice and hot. Go on and crank it up. When it's hot, open it up and throw the flap meat or flank steak on.
Let it sit there for a couple minutes and then flip. Repeat the flip every couple of minutes. This will achieve a more even cook with less graying.
This will cook QUICK. Flap meat or flank steak does not like to be over cooked. This is where your meat thermometer comes in. After about 5 minutes of cook time, insert your thermometer into the thickest part.
These are your cooking temperatures:
MEDIUM RARE: 135°
MEDIUM WELL: 150°
Cook and flip every couple of minutes until you reach about 5° under your desired doneness. Your meat will continue with some carryover heat and raise about 5° as it rests. Remove and let rest for 10 minutes. Slice AGAINST the grain.
Serve in warm tortillas (or fries or nachos) with your favorite toppings!
By all means, you can cook this to whatever temperature you're comfortable with, but in my experience an overcooked flap or flank meat will probably not be great. It will be significantly tougher than say a piece of meat pulled at 120-125°.
Even though you may order your steak medium well or well done, I would encourage you to be brave and pull the carne asada just a little bit before. The meat is so thin, you will not notice at all that the meat is not your usual "steak" temperature.
Another thing is with flap meat especially and sometimes flank, there are varying thicknesses along the length of the meat. So your thin ends might be completely done and your middle might be pink. That works very well for a mixed crowd that likes varying temperatures.