Rinse the boiled crawfish with lukewarm water to remove some of the salt, which will otherwise get concentrated later. Peel all of the crawfish and reserve the tail meat and the shells separately. Get some kid to pull off all the claws from the shells. Put all the claws into a heavy plastic bag. Using a meat mallet, bash the claws enough to break most of them.
In an eight-quart (or larger) saucepan, sauté the onions, garlic, celery, and bell pepper over medium heat until the vegetables are browned at the edges.
Add the crawfish claws, shells and wine, and bring to a boil. When most of the liquid has evaporated, pour the brandy over the shells. If you are comfortable with flaming dishes and have a fire extinguisher nearby, carefully touch a flame to the brandy. Let the flames die out. Otherwise, just let the brandy boil away.
Add the lemon and enough water to cover all the shells. Bring it to a boil, then lower to the lowest possible simmer. Simmer for thirty minutes, spooning out the scum from the top of the pot every now and then.
Strain the stock into another saucepan and discard the solids. Simmer until reduced to about three quarts. Strain through a fine sieve. (At this point, the stock can be refrigerated for up to three days or frozen for later use.)
In a large saucepan over medium-low heat, make a dark roux with the flour and butter, stirring constantly to avoid burning. When the roux is the color of chocolate, stir it into the crawfish stock with a wire whisk until completely blended.
Place the whole crawfish tails in soup plates, and ladle the bisque over them. Add crawfish boulettes (optional) to the bisque at the table.