Choose a nonreactive saucepan just large enough to hold the foie gras completely submerged in its cuisson. Add the cuisson and bring it to a bare simmer.
Prepare the torchon (A):
Moisten a 12 x 12-in. (30 x 30-cm) piece of doubled cheesecloth and lay it on a work surface.
Reassemble the foie gras lobes and place them on the cheesecloth.
Roll the foie gras in the cheesecloth and gently compress it to form as even a cylinder as possible without crushing or breaking the liver.
Tie each end with a piece of kitchen string (A).
Cut 2 or 3 wide bands of cheesecloth and wrap them around the torchon for extra support.
Cut off the excess cheesecloth with scissors, leaving just enough at each end to use as handles with which to pick up the cooked torchon.
Prepare an ice bain-marie large enough to accommodate the poaching pan and a large quantity of ice. A large bain-marie is needed to bring down the temperature of the hot pan quickly.
Lower the torchon into the simmering cuisson and poach it at 200°F (93°C) exactly 90 seconds (see Note).
Remove the pan from the heat and place it in the ice bain-marie (B). Hold it there until the temperature of the cuisson drops to 180°F (82°C). Immediately remove the pan from the bain-marie and let it stand for exactly 2 minutes.
Again place the pan into the ice bain-marie and cool it until the temperature of the cuisson reaches 38°F (3°C). Add more ice to the bain-marie as necessary.
Lay fresh piece of doubled cheesecloth, 12 x 12 in. (30 x 30 cm), on the worktable.
Remove the foie gras torchon from the cooled cuisson and, without unwrapping it, lay it on the bottom edge of the cheesecloth, Roll it up in the second cheesecloth while gently but firmly compressing it into a uniform cylinder (C).
Prepare the foie gras and cuisson for storage:
Transfer the foie gras torchon into a freshly sanitized, nonreactive container just large enough to hold it.
Pour in enough of the cooled cuisson to cover it completely.
Cover the container and refrigerate the foie gras.
Pour the remaining cuisson into a freshly sanitized container, cover it, and refrigerate it.