Pat the pork roast dry with paper towels, then season all sides with the salt, pepper, garlic powder, and onion powder.
Heat the olive oil in a large skillet over medium-high heat. Sear the roast until deeply browned on every side, then transfer it to the slow cooker.
Arrange the sliced onions and apples around the pork. Add the minced garlic over the top.
Whisk together the apple cider, chicken broth, Dijon mustard, apple cider vinegar, brown sugar, rosemary, and thyme until smooth. Pour the mixture over the roast and tuck the bay leaves into the liquid.
Cover and cook on Low for 8 hours or High for 5 to 6 hours, until the pork is tender enough to pull apart easily with a fork.
Transfer the cooked pork to a serving platter and loosely cover it with foil to keep warm.
Pour the cooking liquid into a saucepan. Whisk together the cornstarch and cold water, then stir the slurry into the sauce. Simmer until lightly thickened.
Slice or shred the pork, spoon the warm apple cider gravy over the top, garnish with fresh parsley, and serve immediately.