For the avocado salsa

1. combine all ingredients in a blender. Pulse until smooth and creamy. Add additional tablespoons of water if too thick. You want the salsa to be thin enough to drizzle off a spoon.

For the steak

1. Combine marinade ingredients in a 1 gallon Ziploc bag. Mix until combined. Add flank steak and shake. Remove as much air from the bag as possible, seal and refrigerate for at least 4 hours or overnight.
2. Remove steak from the fridge 30 minutes before searing. Add 1 tablespoon of oil to a cast iron skillet and turn up to medium-high heat. Allow the skillet to become hot first. Place the steak face down and sear undisturbed for 5-6 minutes. Flip and sear for an additional 5-6 minutes. This is for medium, sear for 6-7 minutes per side for medium and higher. If cooking on the grill, cook for 5-6 minutes per side on a grill on medium-high heat with the cover down, roughly 450 °F.
3. Remove steak from the pan and place on a cutting board. Tent with aluminum foil and let rest for 5 minutes before slicing.
4. Find the natural grain running lengthwise through the steak. Cut the steak into thin slices AGAINST/across the grain. This is to ensure the steak is tender and easy to bite. If you slice with or in the same direction of the grain, the meat will be much tougher to bite.
5. To toast the taco shells, place on a greased skillet or pan on medium heat for about 2 minutes per side or until warm and starting to crisp. This will ensure tacos don’t crumble apart. You can optionally steam in the microwave wrapped in a moist paper towel for 20-30 seconds. Toasting is way better.
6. Serve tacos topped with diced white onions, fresh cilantro leaves, and a drizzle of avocado salsa.