Finely dice the black pudding and white bread. In a bowl, whisk together the egg and ½ cup cream. Add the diced black pudding and bread, then season with a pinch of mace, salt, and pepper.
In a separate bowl, beat the egg white with a pinch of salt until soft peaks form. Fold the egg white into the pudding mixture.
Lightly brush 4 small molds or ramekins with truffle oil, then divide the pudding mixture evenly among them. Place the molds in a larger baking pan and fill the pan with hot water halfway up the sides of the molds to create a water bath. Bake at 180°C (356°F) for 25-30 minutes until the tops are lightly browned.
Cut the onions into thin strips. Heat the olive oil in a large pan over medium heat. Add the onions, cinnamon stick, star anise, and juniper berries, then season with salt and pepper. Cook for about 5 minutes until the onions soften.
Pour the balsamic vinegar and rice vinegar into the pan, stirring to deglaze any browned bits from the bottom. Add the lime juice and brown sugar, stirring until the sugar dissolves.
Add the blackberries to the pan and simmer for 3-5 minutes. If the jam becomes too thick, add a splash of water to reach your desired consistency.
Spread the blackberry-onion jam in a thin layer on each of 4 serving plates.
Carefully remove each pudding from its mold by running a small knife around the edges and turning it out onto the jam.