First, take that big old cut of beef out of the fridge and let it relax and get to room temperature. In the meantime, make your rub by mixing the coarse sea salt, ground pepper, mustard powder and nutmeg in a small bowl. Get your barbecue rocking a good medium hot heat. Score the fat on your beef and dry it with paper towels.
Rub in the spice mix, making sure you get it into all the score lines. Rub the beef all over, so the meat is evenly covered, but be careful not to rub too much, so the spices clump together. It should look like a nice thick dusting. Now give the rub some time to dry out the fat slightly, about 5–10 minutes.
First, we need to get that fat layer rendering. We are not cooking the inside yet, just the outside. So place the joint fat side down over direct medium heat. We are looking for a golden crust all over the meat, and nicely rendered fat. Keep an eye on it – you will get flare-ups as the fats break down. A couple of blasts are ok, but you'll need to move the meat around so it's getting a nice even rendering. Keep turning it as well, to sear the meat properly all over and build up that mustardy crust.
It's all about movement. Make sure you pay attention to the sirloin cap and get it cooking but don't burn it. Fat is flavour and we need to get that layer of fat working. Once you have that lovely crust, but the inside is still raw, take it off the heat and let it rest for 10 minutes.
Once it has rested, slice the joint into 2.5cm steaks along the grain. The colour is quite amazing – you've got the crimson claret of the meat matched with the rendered golden fat. Sprinkle some salt on the steaks and off you go. Grill them like a normal steak over direct heat. I like these medium– rare. Once you've got your steaks to where you want them pull them off and serve up. To serve up, slice your grilled picanha steak across the grain to show that sweet fat off.