Many Argentine cooks keep a jar of homemade chimichurri sauce in the fridge (it keeps for several weeks) to either marinate or season cooked meats throughout the week. Chimichurri is always better after a few days, so it is often prepared on a Wednesday or Thursday to be ready for the weekend asado (barbecue). This recipe, from chef Francis Mallmann, pairs in with rib-eye and crunchy Patagonian potatoes. It is an Argentine classic.
A few days before serving, make the chimichurri: In a small nonreactive saucepan, bring the water to a boil and add the salt and red pepper flakes. Remove from the heat and stir until the salt has completely dissolved. Let cool. Add the remaining ingredients and stir to combine. To store, pour into a glass jar, cover, and refrigerate for up to 1 week.
Place the steaks on a large platter and pour over 2/3 cup/165 ml of the chimichurri. Turn the steaks to coat them in the sauce. Let stand at room temperature for 2 hours.
Light a very hot fire in a large charcoal grill/barbecue or parrilla (Argentine grill/barbecue). Just before cooking the steaks, make the potatoes: Peel the potatoes and slice them very thinly on a mandoline or with a very sharp knife; do not place them in water as this will remove the starch and prevent the potatoes from sticking together. Divide the potatoes into 6 portions.
Heat a large cast-iron skillet over medium high heat and add 1 tsp of the butter, tilting the pan to spread the butter as it melts. In the center of the pan, lay one portion of the potatoes in a fan-like pattern, overlapping the slices. Work quickly so as not to burn the butter.
Brown the potatoes on the first side for about 4 minutes, then carefully flip them over with a metal spatula and cook for about 4 minutes on the second side. Sprinkle with a pinch of the salt, then transfer to a baking sheet/tray and keep warm in a low oven while cooking the remaining potatoes. Pour out the butter and wipe the pan with paper towels/absorbent paper. Repeat to cook the remaining potatoes, one portion at a time.
Remove the steaks from the chimichurri and season with salt and black pepper. Grill the steaks for about 6 minutes on each side for medium-rare, or until an instant-read thermometer inserted in the center of the steaks registers 135 degrees to 140 degrees Fahrenheit (57 degrees to 60 degrees Celsius). Serve the steaks over the Patagonian potatoes, with a little chimichurri drizzled on top.