1.In a large deep frying pan, heat oil. Add onions, garlic. Cook, covered, over low heat for about 30 minutes until soft. Stir occasionally.
2.Stir in mustard seeds, cook a further minute.
3.Add sugar, vinegar, salt. Bring to boil, reduce heat. Simmer, uncovered, stirring occasionally for about 5 minutes, or until most of the liquid has evaporated and onions are caramelised.
4.Spoon mixture into hot sterilised jars, seal while hot.
Not suitable to freeze or microwave.Note