Remove the skin of the tomatillo and wash well because they are very sticky. Cut. the onion into quarters. onto a hot barbecue or indoor grill pan add the tomatillos, the Serano pepper, the onion, and the garlic cloves still in their skin. Char on all sides and then add to a food processor with salt and lime juice. Pulse until you achieve a chunky salsa. Place into a container and refrigerate until ready to use.
Remove the flank steak from the marinade and discard the marinade. grill the steak on a barbecue or in a grill pan for 3 to 4 minutes on each side depending on the thickness. Place onto a plate and cover with aluminum foil. Let it rest for at least 10 minutes before cutting.
Cut the steak into thin strips, make sure you cut against the grain for a tender slice.
Heat up your tortilla, add your steak strips and your desired toppings. Enjoy your taco night!