Preheat the oven to 350 degrees F.
In a heavy pot over moderately high heat, warm the olive oil.
Generously sprinkle the osso buco with salt and pepper, then dredge completely in the flour.
Add the onions, leeks, carrots, celery, and mushrooms to the oil in the skillet and sauté, stirring frequently, until softened, about 4 minutes. Push the vegetables to the outside of the pan and add the osso buco. Sear each side of the osso buco until deep golden brown, about 8 minutes total.
Add the wine, sage, rosemary, bay leaf, 2 tablespoons of the parsley, the tomatoes, and stock, and bring to a boil. Cover the pot with a tight-fitting lid and transfer it to the oven. Braise until the meat is very tender, about 2 hours.
Remove the osso buco from the pot and pass the sauce through a strainer. Reduce the sauce on the stovetop. Prepare a gremolata by stirring together the lemon zest and the remaining 1 tablespoon chopped parsley. Serve the osso buco garnished with the sauce and the gremolata.
Serve the osso buco over to a bed of risotto Milanese (with saffron) and enjoy the dish.