Set tenderloin out to come to room temperature about an hour before grilling.

Trim any silver skin by running a sharp knife down the edge to remove the thin layer. 

Tuck the tapered end under and tie the roast or trim and reserve for another use. This will help the roast cook evenly.

Prepare the garlic butter by adding butter and garlic to a small skillet. Cook over low heat until garlic starts to get brown and toasted. Stir in rosemary, salt and pepper. 

Remove from heat and whisk in brown sugar until melted. Set aside and cool slightly. 

Place tenderloin on a sheet pan and pour garlic butter over the meat. Spread the garlic and herbs evenly over the top of the roast.

Prepare a grill fire with hickory for smoke flavor to about 350°. 

Cook tenderloin over indirect fire with the grill lid closed about 30 minutes. Check with a digital thermometer and continue to cook until desired doneness is reached about 45 minutes total for rare, about 1 hour for medium rare. 

Remove from grill to a cutting board and allow to rest about 15 minutes before serving. 

Slice and serve immediately.