Combine vinegar, brown sugar, garlic, and spices to make the marinade.
Trim any obvious fat off the flank steak and place it in a shallow dish or a large resealable bag.
Pour marinade over steak and let marinade for 30 minutes at room temp, or store in the fridge for up to 6 hours. Pull it out 30 minutes prior to cooking to bring back to room temp.
Heat a grill or cast iron skillet to high heat. Spray or drizzle a little avocado oil in the skillet or grill and let it get nice and hot.
Remove your steak from the marinade and give each side a good sprinkle of kosher salt and some cracks of pepper.
Sear for 3-4 minutes per side, depending on thickness. Remove when internal heat hits around 130, since it will continue to cook after you pull it off.
Tent with foil and let steak rest for five to ten minutes before slicing thinly against the grain.