Cut stalks from fennel, saving approximately 1 cup of the fennel fronds. Slice off roots, any damaged parts, and rough fibers from fennel bulbs. Wash and divide each head in half from stem through root end.
Place fennel halves in a large frying pan and cover with broth. Cover pan and, over high heat, bring to a boil. Reduce heat and continue to simmer until fennel is fork tender, about 20 minutes. Move fennel to a 2-2 1/2 quart casserole dish; cut side up, reserving broth in frying pan.
Continue to boil broth until reduced to 1/3 cup. Roughly chop reserved cup of fennel fronds and combine half with remaining broth. Ladle mix ture evenly over fennel.
Combine cheese and bread crumbs and scatter evenly over fennel.
Bake, uncovered, in a 375 degree oven F for 25 to 35 minutes. Cheese should be golden brown and bubbling; fennel should be hot. Sprinkle with remainder of the fennel fronds. Season with salt to taste, if desired.