Here fennel takes the place of leeks or onions, with a subtly different flavor.
Trim the stems off 1 or 2 heads of fennel. Use in stock or discard.
Carefully trim the base of the fennel and then cut around the round piece at the bottom to loosen the outermost layer of the bulb. Repeat with the bulb. Wash all the layers, then thinly slice them crosswise. Toss the fennel with the juice of half a lemon and 1 T. olive oil, then arrange on the bottom of a 2 quart baking dish (not glass) or cast iron skillet.
Thinly slice 2 lbs. of potatoes into rounds. Melt 3 T. butter and toss with the potatoes and 3/4 t. salt plus pepper.
Layer the potatoes atop the fennel. Cover the dish with foil.
Bake for 30 minutes at 450 degrees, then lower the heat to 350 and bake for another 20-30 minutes, until tender.
Grate gruyere cheese to make 1 C. Remove the foil from the pan and sprinkle the cheese over the gratin. Place the dish 4-5 inches below the broiler and broil until the top of the gratin is golden and bubbly, about
4 minutes.