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.
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!