Preheat oven or grill to 400°F.
Add 3 cups peanut oil to a Dutch oven and heat oil to 335°F.
Slice cold steak in 1/8 inch thin slices, set aside.
Cut potatoes into 1/4 x 1/4 inch strips.
Rinse potatoes and pat dry with a paper towel.
Shred Cotija and cheddar cheese.
Divide fries into 4 even groups. Drop first batch in oil, cook 2 minutes, and remove using a fried food skimmer.
Remove fries and place on a paper towel-lined cookie sheet to absorb excess oil.
Allow oil to return to 325°F and place fries back in oil for 5 minutes.
Remove fries from oil and return them to paper towel lined cookie sheet.
Repeat frying steps with each additional batch of fries. As fries come out, place in warm oven.
Season with 1 tsp of kosher salt.
Place fries in cast iron skillet.
Top fries with sliced steak and cheese and place in preheated oven. Cook at 400°F for 5 minutes, or until cheese melts.
Remove from oven. Top with a healthy amount of chimichurri. Serve immediately.