In a bowl, soak the cubes of bread in cream. Set aside.
Blanch the fatback in boiling water for 25 minutes softened.
Chill and set aside.
Sweat the onions lightly in butter, but do not let them brown.
Add the quatre-épices, thyme and salt.
Mix in the flour and cook over medium heat for approximately 2 minutes. Set aside.
In a large bowl, mix the blood, fatback, onions and cream-soaked bread. If necessary, add salt to taste.
Fill the pork casings with cool running water to ensure there are no tears in them. Cut them into sections approximately 3 feet (1 metre) long. Tie one end off with the butcher’s string.
Using a large funnel, pour the boudin mixture into the cashing and tie off the ends firmly. The sausages must be cooked as soon as the boudin mixture is added, as the casings are permeable.
To cook, bring a large pot of water to a boil. As soon as the sausages are placed in the water, lower the heat. You need to cook them over very low heat, ensuring the water does not reach the boiling point. The temperature should be approximately 175F (80C).
After the sausages are in the hot water, pierce them lightly with a toothpick to avoid any air bubbles. Cover the pot and cook for 20 to 30 minutes, depending on the size of the sausages.
When the desired texture is reached, stop the cooking by plunging the sausages in a combination of ice and water. The boudin must be firm but still creamy in the middle. Overcooked boudin becomes grainy.