Combine all of the carne asada ingredients (except the tortillas and cheese) in a large bowl or gallon ziplock bag. Turn several times to ensure that the steak is evenly coated. Allow the meat to marinate for at least 1 hour or up to 24 hours.
Next, caramelize the meat. This can be done on a hot grill, or by placing the meat on a wire rack over a lined sheet pan under a broiler. Grill or broil for 7-10 minutes on each side, or until crisp and deeply browned.Allow the meat to rest, and slice it against the grain. Serve with tortillas and desired toppings.
For the creamy avocado salsa:
Place the tomatillo, jalapeños, garlic, and onion cut-side down on a lined sheet pan and roast at 375°F (190°C) for 25 minutes.Add to a food processor with remaining ingredients, and process on high speed until smooth.
For the pickled onions:
Add the onions to a glass container and cover with the boiling water and vinegar. Add the salt and sugar, and stir to combine. Refrigerate until ready to use.
For homemade corn tortillas:
Add the masa harina to a large bowl, and slowly add in the water while stirring with a wooden spoon. Knead until smooth (the dough should neither fall apart nor stick to your hand), adding more water if necessary. Form into small balls, and press between two sheets of parchment in a tortilla press. Cook on a hot griddle or cast-iron pan for 40-60 seconds per side, and set aside in a tortilla warmer or wrapped in a clean tea towel until ready to use. (They do not keep well, so do not cook more than a few hours ahead of time.)