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.