In a small skillet heat the oil over moderately high heat until it is hot, but not smoking. Fry the shallots, stirring, until they are browned and crisp. Transfer the shallots with a slotted spoon to paper towels to drain, discarding the oil from the skillet, and sprinkle them with salt to taste. The fried shallots may be made 2 days in advance and kept wrapped in the paper towels at room temperature.

 

In a kettle cook the spinach in water and add the fennel seeds so that they cling to the leaves. Cook over moderate heat and stir occasionally, for 3 minutes, or until spinach is wilted. In a colander drain the spinach mixture and press out the excess liquid. In a food processor purée the spinach mixture with 2 Tbsp of the butter, salt, and pepper to taste; transfer the puree to a small bowl. In the small skillet cook the fennel bulb in the remaining 1 Tbsp butter over moderately low heat, stirring, until it is just tender and stir it into the spinach purée. The spinach fennel purée may be made 1 day in advance covered with plastic wrap and chilled.

 

Arrange the oysters in the reserved shells in a jelly-roll pan filled with some of the coarse salt (to balance the shells). Divide the spinach fennel purée among them and bake the oysters in a preheated 400 degree F oven for 12 to 15 minutes, or until they are just cooked through. Top the oysters with the fried shallots, garnish them with the fennel sprigs, and arrange them on a platter filled with more of the coarse salt.

 

To shuck oysters: Scrub the oysters thoroughly with a stiff brush under running cold water. Hold each oyster flat side up on a work surface with the hinged end away from you, insert an oyster knife between the shells at the hinged end, twisting the knife to pop open the shell, and slide the blade against the flat upper shell to cut the large muscle and free upper shell. If the shell crumbles and cannot be opened at the hinge, insert the knife between the shells at the curved end of the oyster, pry the shells open, and sever the large muscle. Break off and discard the upper shell and slide the knife under the oyster to release it from the bottom shell.