Trim and score the fat cap of the Picanha in a shallow crosshatch (do not cut into the meat). Pat dry with paper towels and rub the fat and meat generously with Coarse Sea Salt. Let sit at room temperature for 30 minutes to take the chill off and allow the seasoning to penetrate.
While the meat rests, make the chimichurri: finely chop the Flat-Leaf Parsley, the Red Onion, and the Garlic. Combine them in a bowl with Dried Oregano and a pinch of Red Pepper Flakes for heat.
Add Red Wine Vinegar and Extra-Virgin Olive Oil to the herb mixture, then season with Fine Salt and freshly ground Black Pepper to taste. Stir well and let the chimichurri sit for at least 10–15 minutes so the flavors meld (longer if you have time).
Preheat your grill to high heat (aim for a hot sear zone). Place the Picanha fat-side down over the hottest part of the grill first to render and crisp the fat, about 4–6 minutes, watching carefully to avoid flare-ups.
Move the picanha to a cooler part of the grill or reduce heat and sear the meat side 3–5 minutes per side until you reach your desired doneness (about 125–130°F / 52–55°C for medium-rare). Rotate as needed to develop an even crust.
Transfer the picanha to a cutting board and tent loosely with foil. Rest for 8–10 minutes so the juices redistribute.
Slice the rested picanha against the grain into generous slices and serve with the chimichurri spooned over the top or on the side for guests to add as they like.
