Add the soy sauce, oil, Worcestershire, brown sugar, onion, garlic, salt, pepper, and red pepper flakes to a large zip top bag or air tight container that will fit the tri tip. Mix the marinade ingredients well so it is all combined.
½ cup low sodium soy sauce, 2 Tablespoons avocado oil, 2 Tablespoons Worcestershire sauce, 2 Tablespoons packed brown sugar, ½ cup sweet yellow onion, 2 garlic cloves, 2 teaspoons kosher salt, 2 teaspoons black pepper, 1 teaspoon red pepper flakes