Build a fire in your oven for medium-high heat cooking (450° to 500°).
Quarter and core the fennel, then slice the quarters crosswise ¼-inch thick. Tear the feathery greens into small pieces; set both aside. Combine the wine, fish stock, Pernod, and pepper flakes in a jug. Assemble all the necessary ingredients and tools at the oven.
Warm a large cast iron skillet or a paella pan in the oven. When it’s very hot, add the olive oil and then the chopped fennel, shallots, and garlic. Sizzle for 2 to 3 minutes, rotating the pan once or twice, and season with salt and pepper. Add clams and sausage slices and the wine mixture and simmer until the clams have begun to gap open and there is about ¼-inch of flavorful liquid in the pan, 6 to 8 minutes, rotating the pan as necessary. Swirl in the butter and remove the pan from the oven, swirling the mixture until the butter emulsifies into the juices. Top with the fennel greens and spoon into bowls, distributing the buttery juices equally.