Bring everything to a boil (apple cider vinegar, plum jam, golden syrup, lemon juice, garlic, ginger, cardamom pods, allspice, salt, pepper, onion powder, nutmeg, cinnamon and optional: cayenne pepper) in a small saucepan, then simmer over a medium-low heat for around 20 minutes until reduced and thickened to your liking. Stir occasionally to make sure nothing is sticking.
1 ½ cup apple cider vinegar, ¼ cup plum jam, ¼ cup golden syrup, 1 tbsp lemon juice, 2 garlic, ½ tsp ginger, 2 cardamom pods, 1 tsp allspice, 1 tsp salt, ½ tsp black pepper, ½ tsp onion powder, ¼ tsp ground cloves, ¼ tsp ground nutmeg, ½ tsp cayenne pepper