To make the marinade, mix together all the marinade ingredients in a bowl until well combined and pour into a zip lock bag.
Place the picanha roast into the zip lock bag with the marinade and turn to coat. Place in the refrigerator for 2 to 4 hours to marinate, overnight would be even better.
Bring the roast to room temperature and pre-heat the oven to 200°C/390°F.
Remove from the marinade and season well with salt and pepper.
Pour a splash of oil in a hot pan and lightly brown all sides of the roast, including the ends, to seal the meat and lock in the juices.
Place the beef in an oven-safe dish and roast for 20 minutes.
Reduce the oven temperature to 170°C/340°F and cook for a further 30 minutes for a medium-rare to medium roast beef. Cover the roast with tinfoil if the fat is starting to burn.
Transfer the beef to a chopping board, loosely cover with tinfoil and let it rest for 15 minutes.
Slice the picanha roast and serve with a selection of sides such as roasted vegetables, crispy roast potatoes and a fresh salad.