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Butterfly the marinaded tenderloin. Prepare the stuffing. Spread your stuffing onto the pork. Tie up the stuffed tenderloin. Grill normally. Let rest, then remove the twine.
Take a sharp knife and cut the pork lengthwise, approximately 3/4 of the way through. Open the cut, flattening the meat out as much as possible. If you want, you can take a meat tenderizer and hammer out the pork slightly, to open up the tenderloin to hold all the stuffing. Depending on how much stuffing you make, it might be necessary. You'll end up tying it up anyway, so feel free to flatten it out considerably. Stuffing tenderloin with a mixture of dry bread, cheese, bacon, and vegetables can up the ante considerably. If you want to kick grilled tenderloin up a couple of notches, stuffing it is the way to go. You can experiment with ingredients that you like, or go with a few tried-and-true standards. Bacon-mushroom stuffing: In a skillet, crisp up 3-4 slices of bacon, then add two cups of white button mushrooms, and a pinch of salt. Let the mushrooms sit untouched for one minute, then stir to coat them in the mixture and cook until soft. Add 2 cloves of minced garlic and cook until soft. To this, add 2-3 tablespoons of dried breadcrumbs, and a handful of fresh chopped parsley to complete the stuffing mixture. Italian Spinach stuffing: In a food processor, add a cup of fresh baby spinach leaves, a half cup of fresh basil, two cloves of garlic, and 2-4 tablespoons of grated parmesan cheese. Add a pinch of crushed red pepper, Italian seasoning, and a drizzle of balsamic vinegar to finish the stuffing. Bacon-Wrapped Stuffed Tenderloin: In a bowl, combine a small finely chopped onion, 1-2 stalks of celery, 1-2 slices of dry toast, torn into small pieces, 3 tablespoons of melted butter, and salt, pepper, and oregano. Use this to stuff the tenderloin, then roll it up and wrap the whole thing with 6-8 strips of bacon, around the width of the meat. Tie it and grill it as normal. It's a delicious combo. After you've flattened it out slightly, you can spread an even layer of whatever stuffing you've mixed up over the inside of the meat, then either roll it up or truss it up, depending on how flat it is and how thick a layer of stuffing you've added. If you want to roll up your tenderloin, spread a thin layer of stuffing on the meat, no more than a quarter inch or so. Start rolling the meat on the long flat side, tucking the stuffing on the inside, so it spirals. After you've rolled it up, tie it to keep it together. If you want to keep the tenderloin more whole, don't pound it out, and stuff your stuffing into the butterfly cut as thickly as you want. Wrap it tightly in plastic wrap to get the shape to hold, then you can tie it up with twine before grilling. Wrap a piece of cooking twine around the width of the meat to close up the cut. You don't have to get fancy, just take a few short pieces of twine and tie at least three times around the meat, one in the middle, and one at each end. If you don't have cooking twine, you can use some kebab skewers to pin the 2 sides of the meat together, pinching it closed. After the meat is cooked, remove the skewers and slice normally. Follow the basic instructions for grilling a whole tenderloin, browning on all sides, then cooking at a cooler part of the grill for about 20 minutes. If you've got a particularly-messy stuffing, you might move the tenderloin onto a piece of tinfoil after moving it to the cooler part of the grill, to collect the mess. Take some care when you're testing the temperature of the meat, so you don't stick the reader into the stuffing, getting an inaccurate reading. The internal temperature of the meat, not the stuffing, should be between 140 and 155 degrees F (68 degrees C) according to a meat thermometer. Don't remove the twine right away, or the meat might start to unroll and come apart. Let the stuffed tenderloin rest normally and it'll hold it's shape. After 10 or 15 minutes, remove the twine and slice into inch-thick portions, and serve.