In a small bowl, combine chicken broth and cornstarch, mix until smooth and set aside.
In a 2-quart saucepan over high heat, saute onion in oil until limp. Add chicken broth mixture, milk, nutmeg and pepper; bring to a boil. Remove from heat, add Fontina cheese and stir until melted.
Squeeze liquid out of spinach. Rinse fennel and chop feathery green leaves, reserving 1/2 cup. Trim and discard remaining stalks, root ends, bruised areas and coarse fibers from fennel heads; cut heads crosswise into 1/8" slices. Scrub and peel potatoes; cut lengthwise into 1/8" slices.
Lightly oil a shallow 3-quart casserole dish. Layer 1/3 of the potatoes, overlapping slightly, on the bottom. Cover potatoes with 1/2 of the fennel, 1/2 of the ham and 1/2 of the spinach. Spoon 1/3 of the Fontina sauce over the spinach. Repeat layers. Spoon 1/3 of sauce on top of spinach and top with remaining 1/3 of potatoes. Spoon remaining 1/3 of sauce over top. Bake, covered, at 375 for 30 mins.
Uncover and continue baking until top is richly browned and potatoes are tender when pierced, about 1 hour. Sprinkle with Parmesan cheese, chopped fennel leaves and bake 5 mins. longer. Let stand 10 mins. before cutting. Serves 4-6.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.