In a large zip-top bag or shallow dish, whisk together the molasses, coarse-grain mustard, and apple cider vinegar. Reserve 2 to 3 tablespoons of the marinade for basting before adding the pork.
Season each pork tenderloin evenly with 1 teaspoon of all-purpose seasoning.
Add the pork to the marinade and turn to coat completely. Seal the bag and refrigerate for at least 30 minutes or up to 8 hours.
Remove the pork from the marinade and discard the used marinade. Let the pork stand at room temperature for 15 to 20 minutes while the grill preheats.
Preheat the grill to medium-high heat. Lightly oil the grates if needed to prevent sticking.
Grill the pork for about 20 minutes, turning every 4 to 5 minutes, until an instant-read thermometer inserted into the thickest part registers 145°F.
During the last 10 minutes of grilling, brush the pork with the reserved marinade.
Transfer the pork to a cutting board and let it rest for 5 minutes before slicing and serving.
Christina's Notes
Use a Thermometer: Pork is perfectly cooked when it reaches an internal temperature of 145°F. Don't judge doneness by color alone.
Reserve Marinade First: Always set aside the portion you'll use for basting before adding the raw pork. Never reuse marinade that has touched raw meat.
Rest Before Slicing: Let the pork rest for 5 minutes so the juices redistribute before slicing.
Storage: Store leftover pork in an airtight container in the refrigerator for up to 4 days.