Trim stalks from fennel. Reserve about 1 cup of the feathery green fennel leaves. Trim root ends, bruised areas, and coarse fibers from fennel heads. Rinse, then cut each head in half from stem through root end. Lay fennel in an 11- to 12-inch frying pan; add broth. Cover and bring to a boil over high heat; simmer until fennel is tender when pierced, about 20 minutes. With a slotted spoon, transfer fennel, cut side up, to a shallow 2- to 2 1/2-quart casserole. Boil broth, uncovered, over high heat until reduced to about 1/3 cup. Coarsely chop fennel leaves and add about half to the broth. Spoon mixture evenly over fennel. Mash cheese with breadcrumbs over fennel. Bake in a 375 degrees oven until cheese begins to brown and fennel is hot, 25 to 30 minutes. Garnish with remaining fennel leaves and season to taste with salt.