In a medium bowl, combine the melted butter, maple syrup, Slow Smoke Gold, whisky and salt, and mix thoroughly until the salt is dissolved. Suck up the mixture into a clean kitchen syringe.
Place the tenderloins on a clean work surface. Carefully, inject the pork with the marinade about every inch so the flavour is spread throughout the meat. Transfer the tenderloins to a dish, cover and refrigerate for at least a couple of hours or even overnight.
Apple butter onions
While the pork is infusing, melt the butter in a sauté pan on medium heat. Reduce the heat to low, add the onions and cook for 15 to 30 minutes until softened. Stir in the Apple Butter BBQ sauce, and continue to simmer for another 30 to 45 minutes until the mixture is thick and has the consistency of chutney. Keep warm until ready to serve, or transfer to an airtight container and refrigerate.
Finish pork tenderloins
Prepare your grill for direct grilling, medium heat. Brush or wipe the grates with oil, if necessary. Place the pork tenderloins on the grill and sear them, rolling the loins every few minutes so they colour on all sides.
Remove the pork from the grill when the internal temperature of the meat reaches 145°F. Transfer the pork to a plate and allow to rest for 5 minutes. Leave the grill on.
Arrange the apples on the grill and cook for a couple of minutes, until the fruit is softened and lined with char marks. Remove from the heat.
To serve, cut the pork tenderloin into 2-inch slices and arrange them on a platter. Set a slice of apple in between each piece of pork. Top with a dollop of the apple butter onions.