Add all the marinade ingredients into a plastic bag, close and shake it up really well to combine. Add the flank steak into the plastic bag, close and shake to combine. Refrigerate for 1 hour or up to 8 hours, so the flavors can sink into the meat.
Bake fries following package instructions.
Preheat a ridged grill pan, or an outdoor grill, over medium high heat. Brush the grates with a little oil.
Pull the steak out of the marinade and season it on both sides with salt and pepper to taste.
Grill the steak for 8 to 10 minutes per side, turning once, until steak is well charred on outside and cooked to your liking. Remove steak from the grill and allow to rest for 5 minutes to allow the juices to settle. Slice against the grain into thin strips.
To assemble the fries nachos: arrange a layer of fries on a baking sheet, or a heat-proof platter, evenly distribute cheese sauce over the fries. Broil the fries nachos for 2 to 3 minutes just to melt the cheese. Remove from the oven and add carne asada strips, tomatoes, avocado and green onions. Garnish with the cilantro sprigs.
One of my “secrets” when I make carne asada is not let the meat marinade more than 8 hours, because the steak gets kind of mushy. Carne asada is typically made with skirt steak, but my personal favorite is flank steak, because it’s faster to cook and easier to slice, and it's a leaner. Some recipes out there call for dried or fresh chilies, but I use ancho chile powder because it’s easy to find and convenient.Make sure that you slice it against the grain—that is, perpendicular to the muscle fibers that appear in the meat.