Combine the clams and clam juice in a pot. Bring to a boil, cover, and cook until the clams open, 5 to 7 minutes. Using tongs, transfer the clams to a plate; discard any clams that did not open. Avoiding any sand or grit, scoop out and reserve ¼ cup of the cooking liquid.
Remove the clams from the shells and transfer to a cutting board, then finely chop. Separate the shells and arrange the bottoms on a large rimmed baking sheet; discard the top shells.
Preheat oven to 350°F. In a large skillet, melt the butter in the olive oil over medium heat. Add onion, celery, and garlic. Cook, stirring, until softened, about 5 minutes. Add the chorizo and cook, breaking up the meat, until the fat is rendered, 3 to 5 minutes. Add the cream sherry and simmer until reduced by half, about 2 minutes.
Stir the chopped clams, spinach, heavy cream, and the reserved cooking liquid into the chorizo mixture; cook until hot and the spinach is wilted. Remove from heat and stir in the panko and parsley until well mixed. Season the stuffing with salt and pepper; let cool.
Divide the stuffing evenly in the shells on the baking sheet and cover with foil. Bake until hot, about 20 minutes. Remove foil and bake for about 5 minutes more, until tops are lightly browned. Serve hot or warm.