Preheat oven to 350°

Heat a cast-iron (or oven-safe skillet) until it's very hot.

Slice the meat in half at the center if you prefer. It's easier to handle and will ensure each portion will cook evenly. Tri-Tip is an unusual shape (triangular) so 2 even oval portions are easier to manage while searing and cooking time will be a little quicker and both pieces will cook evenly.

Sear the outside of the beef in a tablespoon of oil in the hot skillet.

Turn the beef while it sears, so that all sides are well browned on all sides.

Place the skillet into the oven for about 25 minutes.

Test the meat with an oven thermometer and remove the pan from the oven when the meat reaches 135° for medium rare.

I prefer my beef medium-rare.