Trim off any excess fat on the surface of the meat. Place roast into a resealable plastic bag.
Chop green onion, reserving 1-2 tablespoons for garnish. Set aside.
Combine soy sauce (or tamari) with brown sugar, sesame oil, garlic, water, vinegar, ginger, and white pepper. Stir in green onions.
Pour mixture over tri-tip making sure to work the marinade into every part of the meat. Release air from bag, seal, and place into the refrigerator for 8-24 hours.
Preheat grill for medium heat.
Remove tri-tip from back and reserve marinade. Place tri-tip on the grill and carefully top with some of the reserved marinade. Either pour it over the top of the meat or place the marinade in a bowl and apply it with a sauce brush.
Close the grill lid and cook for 30 minutes or so, turning the roast occasionally. I recommend medium-rare to medium (140-145 degrees F.) for this recipe.
Once the internal temperature reaches the desired doneness, remove tri-tip from the grill and place onto a clean, sturdy cutting board. Rest the meat for 10 minutes, carve across the grain and garnish with toasted sesame seeds and finely chopped green onion.
If making a sauce, double the marinade recipe. Split the mixture in half and use half as a marinade and place half in a small saucepan. Bring to a boil, reduce heat to low, and simmer for 6-8 minutes. If the sauce is too runny for your liking, simply thicken it with cornstarch (2 teaspoons cornstarch, mixed with 4 teaspoons of water). Serve as a side sauce.