Rinse veal shanks and pat dry. Sprinkle lightly all over with salt and pepper, and place in a single layer in a 12- by 17-inch roasting pan. Bake, uncovered, in a 450° regular or convection oven until shanks are beginning to brown, about 15 minutes.
Meanwhile, rinse and drain fennel. Trim off stalks; chop enough feathery green leaves to make 1 tablespoon (reserve for gremolata, following) and discard remaining greens and stalks. Trim and discard root end and any bruised areas from fennel head. Cut head lengthwise into quarters, then thinly slice crosswise. In a bowl, mix fennel, onion, carrots, celery, and garlic with olive oil to coat.
With tongs, turn shanks. Distribute vegetables around shanks in pan. Bake until shanks are well browned and vegetables are beginning to brown, about 15 minutes longer. Add broth, wine, tomato paste, and sage to pan; stir carefully to mix and scrape browned bits free. Cover pan tightly with foil.
Bake in a 325° regular or convection oven until meat is very tender when pierced, 1 1/2 to 2 hours. Increase oven temperature to 400°.
Uncover pan and gently stir beans into sauce around shanks. Bake, uncovered, until bean mixture is simmering and slightly thickened, 20 to 30 minutes longer.
Spoon equal portions of ossobuco and ragout into four wide, shallow bowls, arranging veal pieces on top of bean mixture. Sprinkle equally with gremolata. Garnish with sage sprigs. Add salt and pepper to taste.
Gremolata. In a small bowl, mix 1 tablespoon minced parsley, 1 tablespoon minced fennel greens (see Ossobuco, preceding), 1 teaspoon grated lemon peel, and 1 clove garlic, peeled and minced.