I used a very simple marinade of garlic, olive oil and pepper coarsely mashed then placed in a bag with the tri-tip for about 1 hour. My roast didn't really have any fat on it at all but if there was fat I would have scored it with a knife to make sure that the garlic paste would flavor the entire roast all the way through.
Seared on the BBQ for about 5 minutes per side, then set over indirect heat to continue to cook until med-rare (around 20 minutes). I let it sit for about 5-10 minutes and then ate it!
It was so juicy and tender. It had a strong beefy flavor and definitely tasted different (better!) than other tri-tip that I have prepared. The grass-fed beef so far is a winner. And it was even better the next day in a Thai beef salad with cilantro, lemongrass, lime and Thai chilies. My husband had a tri-tip sandwich which consisted of a lot of beef and a kaiser roll--nothing else to take away from the taste of the beef!
Can't wait to try other recipes. I don't know if I'll ever be able to go back to the other beef at the grocery store!