Blend all marinade ingredients together until well combined. The better it's blended the more fragrant the marinade will be!
Place steak into large heavy duty gallon ziploc bag. Pour marinade over steak - be sure it's covering all the meat - and tightly seal bag. Place in fridge for at least 4 hours, up to overnight.
Place all chimichurri ingredients in blender except for parsley and cilantro. Blend until well combined and smooth.
Once smooth, add in cilantro and parsley and pulse lightly until herbs are just barely chopped. Don't puree too finely so you can get some of that greenery in each spoonful. Double check the flavor to make sure it has enough salt. If not, add more. If too much, add more lime juice until it's right.
Pour chimichurri into a glass jar or tupperware container and chill in fridge. (You can make this right before the meal or early in the morning and let it chill all day.)
You can cook the steak on the grill or in a nonstick or cast iron skillet, well seasoned.
Heat grill to medium heat, or heat pan on stove to medium high heat.
Once grill or pan is hot, remove meat from marinade with tongs and place onto grill or into pan. Sprinkle with salt and pepper while cooking.
Cook for 8-10 minutes on each side, depending on how done you like your steak. Once you flip the steak, salt and pepper the other side.
Once cooked, remove meat from heat and let rest on cutting board for 5-10 minutes before slicing.
Once rested, slice meat thinly against the grain.
Warm tortillas and build tacos optionally with 2 tortillas each - first with steak, then adding avocado, cheese, red onion, chimichurri, and cilantro.