Combine sea salt, garlic salt, and black pepper and rub over both sides of the tri-tip. If possible, allow meat to marinate in the seasonings for 3 hours and let the meat come to room temperature for about 1 hour before cooking.
Prepare grill by burning several logs of red oak. When coals are hot, starting with the fat side down place trip-tip on direct heat and sear for about 10 minutes on both sides. To finish cooking the meat, raise the grill of a Santa Maria style BBQ or move the meat away from the hot coals. Depending on the thickness of the cut, tri-tip will typically take between 20 to 30 minutes. Use an instant-read thermometer to check the internal temperature: 120-125°F for medium-rare and 130-135°F for medium. (Note that due to the triangle cut of tri-tip, you will have some thinner parts that cook more quickly than the thicker sections.)
Brush with chimichurri butter and allow meat to rest for 10 to 15 minutes in a pan near the heat.
Slice the tri-tip ¼ to ½-inch thick, against the grain and with the knife at a slight angle.
Melt butter and blend in other ingredients.