To make the marinade, place all the ingredients in a bowl and whisk until thoroughly mixed.
If you like things a little spicier, go ahead and add the chili powder as well.
You may be wondering why you've added mustard to a red wine marinade and the answer is simple.
The marinade you're making is a suspension - a mixture of liquid(s) and solid particles that are large enough to sink.
The mustard acts as an emulsifier to help blend together the oil and vinegar as well as to keep your spices evenly dispersed.
Let the marinade sit for 10-15 minutes to allow the flavors to combine before giving it one final round of whisking.
Take your skirt steak and use a sharp knife to score both sides against the grain roughly 1/8 of an inch deep at 1-1/2 inch intervals.
This will help create more surface area for your marinade and make it easier to penetrate the meat completely.
Place your steaks in a gallon ziplock bag and pour in the marinade.
Take about a minute to massage the marinade into the meat, then seal the bag and place in the refrigerator.
It's a good idea to place the bag into a bowl to prevent a mess in case it has a leak.
Let the steaks marinade for at least 6-8 hours, although I prefer overnight.
The longer the marinating time, the more flavor the steaks absorb.
Prep and preheat your grill for indirect heat.
If you've never set up a grill for indirect heat before, don't worry because it's easy.
Indirect heat means there is no heat source directly under the food.
The coals or gas burners on the outsides of the grill are lit and the food is placed in the center.
For this recipe, you will be cooking on direct heat to start so don't put your coals so close to the walls that you can't fit the steaks over them.
While your grill preheats, take the steaks out of the fridge and let them come to room temperature.
This helps to ensure they will cook evenly.
Taking care to save the marinade, remove the steaks and place on direct heat.
Grill for 3-4 minutes per side for rare (internal temperature should be 120 degrees F).
If you prefer medium rare (130-135 degrees F), place the steaks on indirect heat and close the lid for 2-3 minutes.
I don't recommend cooking skirt steak any more than medium rare due to its habit of becoming extremely tough beyond that.
Remove your steaks from the grill and let them rest for 5-10 minutes so the juices redistribute evenly.
Cut the steaks across the grain and serve.