How to Make It
In a large skillet, cook the bacon over moderately low heat until it is lightly colored and most of the fat has been rendered, about 10 minutes. Transfer the bacon to a plate with a slotted spoon.
Pour off all but 2 tablespoons of the fat from the skillet. Add the celery and onion and cook over moderate heat until tender, about 8 minutes. Add the garlic and cook, stirring, until fragrant. Increase the heat to moderately high and add the spinach. Season with salt and pepper and cook, stirring, until the spinach has wilted and any excess liquid has evaporated, about 4 minutes.
Transfer the spinach mixture to a large bowl and add the diced bread, bacon, parsley, thyme and tarragon. Mix well and season with salt and pepper. Let the stuffing cool to room temperature.
Preheat the oven to 425°. Season the cod inside and out with salt and pepper and rub 2 tablespoons of the olive oil all over the outside of the fish. Open the fish and sprinkle the inside with the lemon juice, then fill with the stuffing; press gently to close. Using a sturdy needle and thread or toothpicks inserted vertically, loosely sew or pin the fish together. Transfer the cod to an oiled rack.
In a large roasting pan, toss the leeks and fennel with the remaining 2 tablespoons of olive oil and salt and pepper. Spread the vegetables in an even layer and set the cod on the rack on top. Roast in the center of the oven for 25 minutes.
Meanwhile, in a large bowl, whip the cream to soft peaks. Beat in the horseradish and season with salt and pepper; refrigerate.
Remove the roasting pan from the oven and pour the wine around the fish. Sprinkle with the red pepper and arrange the clams around the cod, hinges down. Return to the oven and roast for about 15 minutes longer, or until the cod is just cooked through and the clams are open; check periodically, removing clams as they open. Cover the opened clams with foil to keep them warm.
When the cod is done, remove the pan from the oven and preheat the broiler. Remove all remaining open clams from the pan; discard any that have not opened. Broil the cod for about 2 minutes to crisp the skin, rotating the pan as necessary.
Cut the fish into serving pieces. Spoon the fennel and leeks onto plates and top with the cod and stuffing. Spoon any roasting juices over the top and garnish with the clams. Pass the horseradish cream at the table.