Season pork tenderloin on all sides with salt and pepper to taste. Pat down the seasoning.
In a large skillet, heat butter and oil over medium-high heat. Add the seasoned roast and sear until golden brown on all sides. Transfer to a large slow cooker.
Add the onion and garlic powder to the pan. Cook, stirring occasionally, until golden, then stir in the tomato paste. Transfer this mixture to the slow cooker.
Add dry red wine to the pan, scraping up the browned bits. Cook for 2-3 minutes until the wine reduces by half, then add it to the slow cooker.
Finally, add chopped carrots, sliced celery stalks, red potatoes, dried thyme, dried rosemary, and bay leaves to the slow cooker, followed by beef stock.
Cook on low heat, about 6-8 hours. During the last hour, place cornstarch in a small bowl. Add broth, and whisk. Pour back into the slow cooker, stirring gently to mix.
Continue cooking on LOW until the sauce thickens. Remove the pork from the slow cooker and shred it with two forks. Discard bay leaves.