Coat the tenderloin with the garlic and kosher salt. Wrap in saran wrap and refrigerate for 1 hour or overnight. Bring to room temperature before grilling, at least 1 hour.
Preheat the grill to 250°F using a Low & Slow Briq.
Brush the garlic off the tenderloin as it tends to burn. Place the meat on the grill and insert your Spark temperature probes, one through the center of each end. For medium-rare, set the desired temperature to 125°F. The cook time should be around 45 minutes, but pay attention to the internal temperature of the meat, not the time.
When the internal temperature of the meat reaches 125°F, remove the meat from the grill and set on the cutting board on the side table; wrap in foil to keep warm. Carefully switch out the Briq to a Quick or Everyday Briq and turn your grill up to 600°F. For this step, we recommend using a heavy duty grill glove or high temperature oven mitt to grab the Briq pan. Then use a metal ash can with a sealable lid or other purpose built ash receptacle to dispose of the coals safely. Do not pour the ashes into a regular trash can or anywhere that can start a fire!
With the new Briq loaded in the grill, turn the control ring on the dial to the left or “off” position for one moment, then ignite the new Briq by holding the control ring all the way to the right for three seconds. After the Halo light starts to blink orange, move the control ring back to the center or “On” position. Once the new Briq gives off a puff of smoke, set the temperature to 600°F and leave the lid open for 5-10 minutes to let the ignition flames die down. Then shut the lid and let the grill rise to temperature.
Sear each side of the tenderloin for about 3 minutes. Remove from the grill, tent with foil and let rest while you make the sauce.
In a saucepan, melt the butter, then add the shallot and rosemary and cook until the shallot is soft and translucent. Add the wine and simmer, reducing the sauce for about 10 minutes.
Slice the meat and finish with a few grinds of black pepper and some coarse sea salt. Arrange on a platter and garnish with rosemary sprigs. Serve the red wine shallot sauce on the side for drizzling.