Peel and halve the Tropea onions, then chop them roughly. Wash and core the plums, then cut into small pieces. Add both to a large saucepan.
In a bowl, combine all three vinegars (balsamic, Moscatello, and date balm). Stir in the sugar and red wine. Pour the mixture over the onions and plums in the saucepan.
Add the ground cinnamon, allspice, and freshly ground long pepper directly to the mixture. Place the whole cloves and black peppercorns in a spice infuser or tea ball, then add it to the saucepan.
Set the saucepan over medium heat and bring the mixture slowly to a boil. Once boiling, reduce the heat to low and simmer gently, stirring often, until the mixture reduces to approximately 1.5 kg (about 2–3 hours).
Remove and discard the spice infuser. Using an immersion blender, briefly blend the mixture until small pieces are just visible (do not over-blend).
Stir in the preserving sugar (250 g, 3:1 ratio). Place the saucepan over medium heat and cook for 3 minutes, stirring constantly to prevent burning.
Pour the hot mixture into clean, hot 230 ml twist-off jars. Seal immediately with clean, hot lids. Allow to cool completely at room temperature.
Store in a cool place and let stand for at least 3 weeks before serving. Use as a condiment for goulash or other dishes.