Preheat the oven to 400 degrees Fahrenheit,placing a rack in the center of the oven.
Thinly slice the potatoes and fennel crosswise; ⅛ inch is ideal.
Coat the bottom of a 9-inch x 12-inch casserole with olive oil. Using half the potato slices, build a layer covering the bottom of the casserole, drizzle with olive oil, and add salt & pepper to taste. Repeat this process with all the fennel and then top with a layer of the remaining potatoes.
Place the casserole in the heated oven and remove the cod from the refrigerator.
Check the vegetables after 20 minutes by sticking a fork in the center to test if the vegetables are getting soft. If not, check every 5 minutes until you can pierce the vegetable bed easily.
Remove the casserole from the oven and place the cod on top of the bed, skin side down. Drizzle olive oil on the fish and season with salt & pepper. Return the casserole to the oven for another 15 minutes or until the fish is flaky.
Remove the casserole from the oven, plate the vegetables topped with the fish and sprinkle on the reserved fennel fronds.