Fry the diced bacon over medium heat until crispy and golden brown. Drain on paper towels and reserve the fat in the pan (you’ll only need about 45ml; you can discard the rest).
Fry the onions, garlic, chilli and mustard seeds together in the bacon fat for about 10 minutes. The onions should just start to colour.
Stir in the sherry and loosen all the cooked bits at the bottom of the pan. Reduce the liquid by half. Add the remaining ingredients, stirring until the sugar has dissolved. Add the bacon back into the pan and cook until thick and syrupy (about 10 minutes). Stir frequently to prevent it from burning.
Spoon the relish into a sterilised jar, seal and cool. Refrigerate overnight to allow the flavours to develop. The relish will keep in the fridge for a week but believe me, it won’t last that long – it’s so delicious spread on a slice of hot bread, as a glaze on grilled pork bangers, or served with a cheese platter.
*Omit the sherry or substitute it with water.