If you are using live snails, place them in a ventilated, covered container in the refrigerator for 1 day. Sprinkle the flour into the container, over the snails, on the second day. On the third day, fill a large vessel with 3 quarts of heavily salted water and submerge the snails inside. Soak the snails for 1 hour, allowing them to disgorge (they will produce a lot of foam and the water will turn milky). Drain off the salt water and rinse. In a large Dutch oven or heavy-bottomed saucepan, combine 2 quarts water, the white wine, onion, celery, carrot, leek, bay leaf, sage, thyme, sachet, and snails. Simmer for 1½ hours, or until the snail flesh is tender, skimming any foam that rises to the surface as needed. Strain the snails, reserving 2 cups of the liquid and discarding the rest. Once they are cool enough to handle, use a toothpick to remove the snails from their shells. Cut off and discard the entrails (tortillon). Submerge the snails in the reserved liquid, cover, and reserve, chilled.