Step 1 In a straight-sided sauté pan, melt 3 tablespoons butter over medium heat. Add the fennel and garlic, and sweat, stirring occasionally, until the fennel is tender, 8 to 10 minutes. Increase the heat and add the Pernod. Continue cooking until the Pernod has evaporated, about 1 minute.
Step 2 Stir in the spinach and salt and cook, stirring frequently, until the spinach wilts and any liquid from the spinach has evaporated, 6 to 8 minutes. Stir in the heavy cream and cook until the cream thickens and coats the spinach, 1 to 2 minutes. Remove from heat and add the remaining 2 tablespoons butter, stirring until the butter emulsifies with the cream. Taste and adjust the seasoning if needed, then remove to another dish to cool. The Rockefeller topping can be made up to 1 day in advance, covered and refrigerated.
Step 3Before finishing the dish, heat the oven to 350 degrees. Shuck the oysters and lay them on a bed of rock salt on a gratin dish. Cover each oyster with approximately one-half tablespoon of the spinach mixture. Sprinkle fresh bread crumbs over each oyster, then top with a light drizzle of melted butter. For a better crust, top with a second layer of bread crumbs, gently pressing them into the butter to moisten. Bake the oysters for 3 minutes if using room-temperature Rockefeller mixture, 4 minutes if the mixture was refrigerated. Remove and finish the oysters under the broiler until the crust is nicely browned, 1 to 3 minutes.