Season the roast liberally with salt and pepper. Place roast in the slow cooker and spread the onions, carrots, mushrooms, and garlic around the roast. Pour in the broth and add in the dried thyme and parsley.
Secure the lid on your slow cooker and cook on low for 6 to 8 hours, until beef is very tender. Transfer roast to a cutting board and allow it to rest while you're preparing the gravy.
Pour the juices from the crock pot into a saucepan (you can use a slotted spoon to remove the vegetables and place them in a bowl for serving). Heat the juices to medium-high and bring to a gentle boil. Add the arrowroot flour and whisk well until gravy has thickened, about 3 to 5 minutes. Taste for flavor and add sea salt to taste.
Cut the bison roast into chunks for serving.
Place rutabaga in a large pot and add enough water to cover. Bring to a boil and cook until tender, about 20 to 30 minutes.
Drain rutabaga and return it to the pot along with the remaining ingredients. Mash with an electric mixer or masher until rutabaga reaches desired level of creaminess (note: you can also mash the rutabaga in a blender for the creamiest consistency). Add in the parsley and season to taste with sea salt.
Serve bison roast with slow cooked vegetables, a drizzle of gravy, and mashed rutabaga.
*Omit the onion and garlic for Low-FODMAP