Heat olive oil and butter in a large skillet over medium heat. When melted, add the onions in a single layer and season with kosher salt and freshly ground black pepper. Sauté for about 10 minutes, stirring occasionally, until the onions are golden and caramelized. Remove the onions from the pan and set aside.

While the onions are caramelizing, season the pork roast on all sides with the 1 1/2 teaspoons salt and 1/2 teaspoon black pepper. When you have removed the onions from the pan, increase the heat to medium-high and add the pork roast (you may need to add another teaspoon or so of oil). Sear on all sides.

When the roast has seared, place it in a slow cooker. Top with caramelized onions. In a 2 cup measuring cup, whisk together the apple cider, honey, and mustard. Pour it over the roast and cook on high for 4-6 hours or on low for 8-10 hours or until the meat can easily be shredded with a fork.

When the meat is done, remove the roast and onions from the slow cooker and shred. Pour the cider mixture into a small saucepan and bring to a boil over medium heat. While the cider is heating, whisk together the cold water and cornstarch. When the mixture is boiling, whisk in the cornstarch mixture and stir until thickened. Remove from heat and add vinegar.

Serve the pork on top of mashed potatoes and topped with gravy.