Trim as much excess fat as possible from the pork loin. Put the meat in a vacuum-sealable bag, remove the air and seal the bag. If you don't have a vacuum sealer, you can use the water displacement method to remove as much air as possible. Set the cooker to 140 °F (60 °C) for 3 hours.
In the meantime, make the sauce. Heat a saute pan and add the butter. When the butter is melted, add the red onions. Separate them as much as possible. When the onions are translucent, add the cranberries. Lower the heat to low and cover the pan. You will likely hear the cranberries popping as they cook, for about 20 minutes. When the cranberries have all popped and started to soften, add the cherries. Cook the cherries for about 5 minutes then add the wine. Let the mixture continue to cook on low until the cherries are very soft, about 10 minutes, stirring occasionally. Set the pan aside.
When the pork is done, remove it from the bag and wipe it down with a paper towel. Wet food does not sear; it boils so it is important the pork's surface is dry as possible. If you have other food that is not ready yet, you can put the pork, uncovered, in the refrigerator which will dry it even more.
Heat a cast iron skillet to very hot and smoking. Spray it with olive oil. Add the pork and sear it on all sides, turning it about every 30 seconds so each side gets a lovely sear. Remove the pork from the pan and set it aside. Do not cover it.
Add the fruit mixture to the pan and scrape up any fond. Set the saute pan aside as it will be used later. Stir the fruit mixture in the cast iron pan and set it aside.
Put the saute pan on low heat. Add the balsamic vinegar to the saute pan and scrape the bottom. When the bottom is clear and all the fond has dissolved in the vinegar, taste it for seasoning. If it is too sour, add a teaspoon of sugar. Then add the butter and swirl it around until it has emulsified with the liquid. Put the pork in the sauce and move it around so all sides get a chance to get a little sauce. Add the fruit mixture to the pan and let it turn the pork in that, too. Time for plating.
Cut the loin into serving pieces. Each person would get 1/2 pound of pork, so four pieces.
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