1. In a large bowl, whisk together apple cider vinegar, olive oil, honey, mustard, garlic, salt, red pepper flakes, and pepper. Pour mixture into a large zip-top plastic freezer bag. Add the pork tenderloin. Seal the bag and make sure the pork is covered in the marinade. Chill in the refrigerator overnight, turning occasionally.

2. Remove the pork from the refrigerator and let come to room temperature before grilling. Discard marinade. Preheat grill to medium-high heat. Grill tenderloin, covered with grill lid, 8 to 10 minutes on all sides or until a meat thermometer inserted into thickest portion registers 150° to 155°. Remove tenderloin from grill, and let stand 10 minutes before slicing.

3. To make the honey mustard sauce, whisk together honey and Dijon mustard. Slice pork and serve warm with honey mustard sauce, if desired.