To prepare the lemon grass, peel off the woodiest parts. Pound with the back of a knife, then chop.
Most big city grocery stores carry lemon grass, but in other parts of the country, I've seen it sold as a tubed puree, displayed in the produce department. If you can't find it, you can add some lemon juice and zest to adjust.
You can grill, smoke, or roast the tri tip roast to an internal temperature of 130 to 135 degrees F (54 degrees C).
You can also serve this with a Peanut Dipping Sauce.