Prepare the Carne Asada according to recipe instructions (heads up! It needs to marinate for 4 hours prior to cooking). Once it has rested at least 10 minutes, thinly slice it across the grain, then dice into bite-sized pieces.
Just before serving, scoop the avocados into a bowl. Juice in 1/2 the lime (cut the remaining half into wedges for serving) and add the salt. With a fork, roughly mashed to combine (if you are using Homemade Guacamole, skip this step). Taste and adjust the seasoning as desired.
Warm the tortillas in a dry skillet, or by blistering over a gas stove. For each taco, spread a tortilla with a scoop of the avocado, then top with the asada.
Add the onion, cheese, and any additional desired toppings. Enjoy immediately
Notes
TO STORE: Store leftover carne asada and toppings in separate airtight containers in the refrigerator. The meat and toppings will last 3 to 4 days, but the avocado mixture should be used within a day or so.
TO REHEAT: Reheat the carne asada in a pan over medium heat until heated through. Warm tortillas on a grill or pan, then assemble.
Nutritional information was calculated with .75 pounds flank steak meat, avocados, and lime; does not include tortillas or toppings since there is so much variation.