Toss the onions with the red pepper and the oil, then fry in a frying pan for 5-8 mins over a high heat until lightly charred and softened. Remove from the pan and set aside.
Put the chilli, garlic and ginger in the pan with half the vinegar. Bring to the boil then simmer for 2-3 mins. Add the onions and pepper plus the remaining vinegar, all the sugar, spice and 1 tsp salt.
Bring to the boil then bubble away for about 5 mins until thickened. Add redcurrants and simmer for about 5 mins more, or until they have burst, but still have some shape and the liquid is syrupy.
Remove and pour into a large heatproof jar. Cover and seal while hot. Keeps in the fridge for up to 3 weeks.