For the broth: Remove and reserve the bone from the steak and cut the meat into 2-inch pieces.
Heat a large 6-quart pot or Dutch oven over high heat. Add the steak, steak bone, parsley stems, if using, carrots, celery and onion. Add 8 cups water. The water should cover the contents of the pot; add a small amount more if necessary.
Bring up to a boil, and then reduce to simmer. Simmer uncovered for 1 hour 30 minutes, skimming and discarding any fat and scum that may accumulate on top.
Strain the broth, reserving the meat. Discard the vegetables. To serve the broth on its own, ladle into a mug or serving bowl and add 2 to 3 tablespoons Chianti per serving and some salt and pepper. Serve hot. For the stew, reserve 2 cups of broth, or more as needed.
For the beef stew: Place the potatoes in a 4-quart pot and add enough water to cover by 2 inches. Cover and cook over medium-high heat until the potatoes are cooked through, about 25 minutes. Test by piercing with a knife; the potatoes should have no resistance. Drain and set aside.
Heat the olive oil in a large saute pan over medium heat. Add the garlic and sage and cook until fragrant, about 5 minutes. Add the onions and saute until soft, and then add the crushed tomatoes and cook until reduced and very thick, about 10 minutes.
Dredge the boiled beef pieces (they should still be roughly in 1- to 2-inch pieces) very lightly in flour, shaking to remove all excess. Add to the sauce along with the cooked potatoes and 2 cups of the beef broth. Cover and cook for another 15 minutes to meld the flavors and thicken the sauce. The meat will shred a bit as you stir gently. If the stew becomes too dry at this point, add enough beef broth or water to reach the desired consistency, and bring up to a simmer.
Season the stew with salt and pepper and serve warm with sliced bread.
The broth can be made in large batches and refrigerated for up to 1 week or frozen for up to 1 month. Remove any additional fat that rises to surface of the broth after chilling before reheating and serving.