In a large pot, steam the spinach, using only the water clinging to its leaves. Drain and place on a plate to cool. Squeeze the water out of the spinach and chop coarsely.
In a large pot, melt the butter over medium heat. Add the onion and fennel and sauté until the onion is translucent. Add the white wine, turn the heat to high and boil until the wine is reduced by half. Add the whipping cream and boil until the mixture is reduced to 1 1/2 cups (360 ml). Stir in the salt, tarragon and lemon juice. Remove from the heat.
Cover the potatoes with water, bring to a boil and cook until the potatoes are very tender, 15-20 minutes. Drain and return to the pot. Mash the potatoes until smooth. Beat in the buttermilk, then the butter and horseradish. Season with salt and pepper.
Preheat the oven to 425°F (220°C). Place the oysters in an 8-by 8-inch square (20-by 20-cm) baking dish and sprinkle with the breadcrumbs. Stir the spinach into the sauce and pour over the oysters. Spoon the mashed potatoes over the sauce and sprinkle lightly with paprika. Place in the oven and bake for 20-30 minutes, until the mixture is bubbling and the potatoes are lightly browned.