In a medium bowl, whisk the brown sugar, whole grain mustard, apple cider vinegar, cumin, black pepper, smoked paprika, garlic powder, and kosher salt. Reserve ⅓ cup of mixture to drizzle over turkey right before serving.

Place turkey tenderloins in a medium bowl or pan that is low and wide. Pour remaining brown sugar and mustard mixture over the top and turn tenderloins to coat completely. Cover bowl/pan with plastic wrap and refrigerate tenderloins for 1 hour. Remove from refrigerator, uncover, and let sit on counter while heating the grill.

Heat grill to high heat. Make sure grates are scrubbed clean and then oiled. (To oil the grates, add some canola oil to a small bowl. Then fold a heavy paper towel a few times to make a smaller square. With a tongs, grasp the folded paper towel and dip it into the oil until the paper towel is drenched. Then run the paper towel over the grates, repeating until all grill grates are thoroughly oiled.) Then turn heat down a bit, to medium-high. Place "beauty" side of each turkey tenderloin face-down on the grill grates. Sear for about 4 minutes, until there are beautiful charred grill lines. Using a metal turner, carefully turn each tenderloin over, taking care to not tear the turkey surface by gently working the turner to scrape and lift turkey from grates. Reduce heat to medium and grill for about 8 minutes. Rotate tenderloins on same side (don't flip them over - you want to preserve those pretty grill marks!) after 5 minutes. Then reduce heat to medium low and grill until internal temperature of thickest part of tenderloins reaches 160° F. Grill time should be about 40 minutes total, for tenderloins that weigh about ¾ pound each. Adjust all times for tenderloins that are larger or smaller.

Remove tenderloins to a platter to rest for 5 minutes before slicing. Slice tenderloins and then drizzle reserved brown sugar and whole grain mustard mixture over the top. Serve hot to warm.