Preheat the oven to 140°C (275°F/Gas 1). Dice the fat into small pieces. Place half in a frying pan over medium heat and cook until rendered. Add the onion, quatre épices and salt and cook for 5 minutes. Remove from the heat and allow to cool slightly. Stir through the cream, breadcrumbs and remaining diced fat. Stir through the pig’s blood.
Pour into a 30 × 25 × 6 cm (12 × 10 × 2½ inch) tray lined with baking paper. Place in a bain-marie and bake for 1 hour. Remove from the oven and bain-marie and allow to cool. Remove the boudin noir from the tray and cut into 6 equal portions. Reheat in the oven before serving.