Candied-Fennel-Topped Lemon Cake

Lightly oil pan and line bottom and side with a large round of wax paper, pleating sides and trimming to fit. Lightly oil paper. Line side with a 2-inch-wide strip of wax paper long enough to wrap around inside of pan to cover pleats, then lightly oil.
Cut fennel bulb lengthwise with slicer into enough 1/4-inch-thick slices (about 9) to cover bottom of cake pan.
Cover fennel with cold water in a medium saucepan and bring to a boil. Drain fennel and set aside. Add sugar, water (3/4 cup), zest, and fennel seeds to saucepan and bring to a simmer, stirring until sugar has dissolved. Add fennel slices and very gently simmer until tender and translucent and liquid is syrupy, about 40 minutes. Lift fennel slices out with a fork and arrange decoratively in bottom of cake pan. If you have more than 1/3 cup syrup, boil to reduce; if less, add water. Cool syrup slightly, then pour (through a fine-mesh sieve if desired) over fennel.
Preheat oven to 350°F with rack in middle.
Whisk together flour, baking powder, baking soda, and salt. Beat together butter and sugar in a large bowl with an electric mixer at high speed until light and fluffy. Add eggs 1 at a time, beating well after each addition, then beat in zest.
At low speed, mix in flour mixture in 3 batches, alternating with buttermilk, beginning and ending with flour mixture, and mixing until just combined. Gently spoon batter over topping, spreading evenly.
Bake until cake is golden-brown and a wooden pick inserted into center of cake comes out clean, 25 to 30 minutes. Cool cake in pan 15 minutes, then invert onto a plate and continue to cool. Serve warm or at room temperature.