In a large skillet add the butter and heat over medium high heat. Add the sliced onion and cook in the butter until browned, about 5 minutes. Remove the onions to a crock pot.
Salt and pepper the pork loin on all sides to taste, sear in the same skillet as the onions were cooked over medium high heat on all sides. About 1 minute per side. Add to the crock pot with the onions. Place the onions around and on top of the pork.
In a bowl whisk together the apple cider, apple cider vinegar, mustard, chicken bullion, and garlic and pour over the pork. Cover and cook on LOW 5-6 hours or HIGH 3 hours until the internal temperature reaches 145 degrees F.
Place the pork and onions on a serving platter. Carefully strain the juices and ladle them in a medium saucepan.
In a small bowl whisk together the cold water and cornstarch. Pour it into the juices and turn the heat to medium high. Stir gravy until thickened and pour over the pork or serve on the side. Serve while still warm.
