Place the onions, carrots, and parsnips in the bottom of the slow cooker. Season with salt and pepper.
Rub steak seasoning into both sides of the roast.
Heat oil in a heavy bottomed pan over medium heat. Once oil is hot, add roast and brown for 4-5 minutes on each side. Remove the roast and put it in the slow cooker on top of the vegetables. Pour 1/4 cup of the beef broth into the pan and use a spatula to scrape up all the brown bits. Pour the broth and bits into the slow cooker.
Whisk together the remaining beef broth, balsamic vinegar, and tomato sauce and pour it over the beef in the crock pot.
Cook on low for 6-8 hours or until beef is fall apart tender.
Remove roast to a plate and cover with foil to keep warm. Serve the beef as is or use the following steps to make a gravy.
For the gravy, place 2 cups of the sauce mixture in the crockpot in a small saucepan over medium high heat. Add dissolved cornstarch and bring to a boil. Once boiling, reduce heat to medium and allow sauce to cook, stirring occasionally, until thickened. Serve gravy over meat and onions.