1
In a bowl, whisk together the vinegar, mustard seed, salt and sugar until dissolved. Fold in the onion and cucumber and rest while you prepare the remaining ingredients.
2
In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the scallions and season to taste.
3
Add all of the chimichurri ingredients except the oil to a blender. Purée until smooth. With the motor running, drizzle in the oil until emulsified. Season to taste.
4
Preheat the grill or a large skillet over high heat. Drizzle with oil or brush the steaks with oil if grilling.
5
Pat the steaks dry and season liberally with salt and pepper. Grill the steaks for 3 to 4 minutes per side to sear, then lower the heat to medium and cook until an instant-read thermometer reaches 130°F for rare, 140°F for medium rare and 150°F for medium-well.
6
Transfer the grilled steak to a cutting board, let rest for 5-10 minutes, then slice against the grain.
7
Divide the mashed potatoes between plates. Serve with steaks, pickled cucumbers and chimichurri sauce.