A quintessential grilling cut, the tri-tip comes from the sirloin, which is where the animal's back leg meets the loin. It's a pretty complex muscle as it's grain structure runs in three different directions, hence the name tri-tip. The North American Meat Institute is pretty clever. You can cook the whole puppy as one piece, but could also cut it into two or even three to make it a bit more manageable. To start, trim back the fat to your liking - or don't trim it at all - and take off whatever "silverskin" you can see. Salt it liberally, get your cast iron hot as hell, and bring your oven up to 350°F. Plop the meat in and get a really really hard sear on it, a good 3 minutes per side. Finish it off in the oven to your liking, although we prefer them cooked to a good medium-rare to medium. Or if you are fortunate enough to own a grill, disregard everything and do that! There's plenty of recipes out there, but here's a few we love:
This recipe is great if you have a grill.
Here's one for oven roasting.
If you have a paid subscription to the NY Times, here's an excellent recipe that can be used with or without a grill.