Score the the fat of the meat without cutting to the meat.
Seal the meat on top and bottom, fat side in a large skillet on a high heat.
Strip the leaves off the rosemary and blend the salt with the rosemary and black pepper in a food processor (simply mix with a spoon or pestle and mortar).
Massage the meat with the seasoned salt.
Discard the excess
Bake 200°C (medium high) for 20 minutes with maximum fat side up. Should read 56°C (130°C-135°F) inside.
Remove from the oven, cover the meat with aluminum foil and let it rest for 30 minutes.
Slice the meat against the fibers to ensure more tenderness.
Notes & thoughts
The cut of Picanha is really hard to find, closest thing I've had in the UK is a point of rump. You need to order this in advance. Sometimes mixed results with the rarity of the meat, but this seems to do rare/Medium-rare quite well.