Set a large Dutch oven or heavy skillet over medium-high to high heat. Sear both sides of roast until golden-brown, about 10 minutes per side. Transfer the roast to a 6-quart crockpot.
Reduce heat to medium. Add the onions and/or other vegetables and cook until starting to soften, about 5 minutes. Pour in a few tablespoons of chicken broth and bring to a boil, scraping up any of the brown bits from the bottom of the pan. Pour the onions and pan juices into the bowl of the slow cooker.
In a separate bowl, whisk the remaining chicken broth, mustard, maple syrup, vinegar, paprika, salt, and pepper until combined. Pour the liquid mixture over the roast. Cover and cook on the LOW setting until the meat falls apart and is meltingly tender, about 8 hours. If you only have 6 hours, you can do this 3 hours on high and 3 hours on low and it will still turn out fine. If you only have 2 hours, see Dutch oven directions below.
Transfer the roast to a serving bowl or platter and let it rest for 5-10 minutes. Cut the roast into chunks or shred the meat and add back to the liquid. Serve over rice, potatoes, or bread.