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.
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.
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.
Preheat the grill or a large skillet over high heat. Drizzle with oil or brush the steaks with oil if grilling.
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.
Transfer the grilled steak to a cutting board, let rest for 5-10 minutes, then slice against the grain.
Divide the mashed potatoes between plates. Serve with steaks, pickled cucumbers and chimichurri sauce.