Mix all marinade ingredients using a whisk, until sugar and salt are dissolved.. Set aside 1 cup of marinade to drizzle on top of your meat during and after cooking.
Marinate 4 pounds flank steak in two cups of the marinade for two hours. Two zip lock style bags work well here. Simply place the meat in the bags, pour 1 cup of the flank steak marinade in each bag, squeeze the air out, squish the marinade around several time to distribute and refrigerate.
Preheat grill to 500 degrees. Remove the steak from the marinade and let excess drip off. Reduce the grill heat to 400 degrees. Cook the Flank Steak, about 5 minutes per side. Skirt steak, a much thinner cut of meat, will only take about 2-3 minutes per side. You want a good sear on the outside and medium rare on the inside.
I like to drizzle on a little of the reserved marinade once I place the steaks on the grill and again when I turn them.
About half way through the cooking time, place vegetables in a grill basket and add them to the grill. I like to drizzle them with a little olive oil to help the browning process.
Using tongs, toss the vegetables often to get a good char. If using plum tomatoes, cut them in half and cook them cut side down, without turning. If they get charred before the other veggies, just move them to a cooler part of the grill.
Remove the meat from grill, tent with foil and let rest for 10 minutes.
Slice meat as thin and you can and against the grain, drizzle with some reserved marinade and serve with your favorite accompaniments like a “build your own fajita taco.”
Serve alongside grilled vegetables.
Serve flank steak fajitas with warmed flour or corn tortillas, black beans, Spanish rice, pico de gallo and guacamole.