Step 1

Place garlic, lemon grass (bruised and chopped--see note), grated gingerroot, grated onion, fish sauce, rice vinegar, soy sauce, coriander, cumin, turmeric, cayenne pepper, brown sugar, and vegetable oil in a large mixing bowl. Whisk until ingredients are thoroughly mixed.

Step 2

Transfer tri tip roast to marinade. Poke both sides of the roast numerous times with the tines of a fork to get the marinade into the roast. Cover with plastic wrap. Refrigerate 2 to 12 hours. During the marinade time, remove the meat from the fridge occasionally to turn and poke some more with a fork.

Step 3

Transfer roast to a paper-towel-lined tray to drain briefly. Reserve marinade.

Step 4

Preheat covered grill to 325 degrees F (165 degrees C). Lightly oil the grate.

Step 5

Cook roast covered, over indirect heat, basting with marinade and turning occasionally. Cook about 35 to 45 minutes, depending on how hot your grill is. An instant-read thermometer inserted into the center should read 130 to 135 degrees F (54 degrees C). Transfer roast to a cutting board and let rest at least 20 minutes before slicing.

Step 6

Place any remaining marinade in a saucepan; bring to a boil. Simmer for 1 or 2 minutes. You can use this as a serving sauce.