Mix the spinach, the bread, and the butter together well first, mashing with a fork, before adding the other ingredients of the stuffing and stirring well. Stuff the fish, pressing filets and stuffing firmly together so as to re-form the fish in the original shape. Place it on its side on the tabletop.
Arrange about 20 of the largest and most perfectly formed lettuce leaves in a saucepan, add salt, pour over boiling water, return to a simmer for a minute or so, and drain, pouring carefully into a sieve or colander so as not to damage the leaves or bunch them up into a mass. Run a bit of cold water over to cool them and delicately, one by one, lift them out and spread them on a towel to drain.
Wrap the fish from head to tail (muzzle and tail tip remaining exposed): First salt and pepper the fish’s surface; arrange overlapping leaves of lettuce the length of the belly section, the rib ends gently tucked beneath the fish and the fragile leaf extremities pressed to the surface. Repeat the process along the length of the back, tucking rib tips under and pressing the leaves well into place. Turn the fish over, placing it in an elongated buttered gratin dish, the bottom of which has been scattered with chopped shallots; salt and pepper the newly exposed surface and repeat the performance with the lettuce leaves so that, when finished, the fish is firmly wrapped, mummy-like.
Dribble the dry vermouth over its surface, pour the white wine into the bottom of the gratin dish, adding the section (or broken sections) of bone that was removed from the fish, line the surface of the fish with dabs of butter, press over lightly a sheet of aluminum foil (or buttered parchment), and bake for about 30 minutes at 400° to 425°, basting regularly during the last 15 minutes. Transfer, with the help of spatulas (first detaching muzzle and tail tip from sides of dish if they are touching) to a heated serving dish, discard the bones, pour the juices into a small saucepan, reduce to a light syrup, add the cream, and reduce at a high boil, stirring, by approximately half or until the liquid takes on the consistency of a light sauce (whisk in a couple of tablespoons of butter away from the heat, if you like), and pour it over the fish. Cut into cross-sections for serving.