In the bowl of a slow cooker, combine the onions, garlic, carrots, parsnips, mushrooms, tomatoes, and sage.
Season the short ribs with 1 1/2 teaspoons salt and 3/4 teaspoon pepper and nestle them among the vegetables. Add the wine.
Set the slow cooker to high and cook until the meat is tender and falls from the bones, 8-9 hours.
Using a large spoon, skim off the excess fat and discard. Divide the ribs and vegetables among individual bowls and spoon the sauce over the top. Serve with the crusty bread.