Peel and core the pear and apple. Dice onion, apple, and pear into small pieces.
Preheat a medium cooking pan over medium heat and melt butter. Add diced onion, pear, and apple, and sauté them until soft.
Transfer it into the mixing bowl and mix with minced fresh herbs, some salt and pepper, and grated Gruyere cheese.
Butterfly the Pork:
Take tenderloins out of the package and pat them dry with a paper towel. Place the tenderloins on a large cutting board and remove as much silver skin as you can, if there is some still attached.
Use a sharp knife to cut the tenderloin through the center, lengthwise. Cut almost all the way to the other side, but NOT all the way in half! The goal here is to be able to butterfly the tenderloin, or open it like a book.
To even out the thickness of the meat: Cover opened pork tenderloin with a sheet of plastic wrap. Use the flat side of a meat tenderizer to pound it. Make sure the whole piece has an even thickness of about 1/2 inch. Remove the plastic wrap and discard it.
Stuff, Roll, and Tie the Pork:
Season inside of the pork with some salt and pepper. Divide the mixed filling in two portions and spread each portion over each tenderloin. Spread it out as evenly as possible, leaving about 1/2 inch of meat around the perimeter without the filling.
Roll up the tenderloin lengthwise and place it seam down on the cutting board. Use cooking twine to tie the tenderloins all along the whole length in 1-inch intervals. Make sure not to tie too tight where it starts cutting into meat. Season on the outside with a little salt and pepper and well.
Smoking the Pork:
Set up your smoker and use milder wood chips like maple, pecan, apricot, apple, or peach. Preheat the smoker to 225° and make sure the grate is clean.
Place the tenderloins onto a lightly greased, mesh grill basket. Insert the leave-in meat thermometer probe into the center of the thickest part of the larger tenderloin to safely track the temperature. (Make sure to insert the thermometer into the meat and not the stuffing. You can usually feel for the meat with the thermometer probe because when you are inserting the probe into the meat, it will feel more dense and have some resistance. The filling will feel very soft.)
Place on the smoker, close the lid, and smoke until internal temperature reaches 145° (2-3 hours).
Take off the smoker and place the tenderloins onto the cutting board. Let them rest 5 minutes and take off the twine. Let them rest another 5-10 minutes before slicing.