Prepare the pork: Trim excess fat from the pork roast. Pat dry with paper towels, then season all sides with salt and pepper.
Layer the slow cooker: Place the sliced onion and minced garlic in the bottom of the slow cooker.
Make the sauce: In a bowl, whisk together brown sugar, balsamic vinegar, rosemary, and thyme.
Add the pork: Place the seasoned pork roast on top of the onions and garlic. Pour the balsamic mixture evenly over the pork.
Cook: Cover and cook on low for 6–8 hours or on high for 3–4 hours, until the pork is fork-tender.
Shred the pork: Remove the pork from the slow cooker and use two forks to shred it.
Make the glaze (optional): Pour the cooking liquid into a saucepan. Simmer over medium heat until slightly thickened, about 10 minutes.
Serve: Return shredded pork to the slow cooker, toss with glaze, and serve hot.