Add all marinade ingredients to a large bowl and whisk to combine.
Place tri-tip in a large storage bag and pour marinade over the roast. Seal bag and gently shake bag around to fully coat the roast. Place in the fridge for at least 8 hours but ideally 24 hours. Pull roast out of the fridge about 30 minutes before cooking.
Preheat oven to 400 degrees.
Remove roast from bag, leaving marinade drippings in bag for later.
Heat a cast iron skillet to high heat. Once very hot, add oil to the skillet and begin to sear the roast on all sides for about 2 minutes until there is a nice char.
Transfer roast to an oven rack on a baking sheet. Bake for 20-30 minutes depending on your preferred doneness. 130 degrees for rare, 140 degrees for medium-rare, 145 degrees for medium. You will need a meat thermometer to check the temperature.
While the roast is in the oven, pour remaining marinade drippings from the bag into the same skillet you seared the roast along with a 1/2 cup of water and bring to a low simmer for about 5 minutes.
Once done, remove roast from oven and let rest for 5-10 minutes. Thinly slice roast and pour sauce on top. Sprinkle with chopped green onion and sesame seeds(optional). Enjoy!