Place the mustard seeds in a small bowl and cover with the vinegar. Allow them to soak overnight to ensure they are nice and soft and blend through the mixture well. (If you don’t have have time to soak them, dry roasting them in a fry pan for a few minutes also works to open.
The next day, place all the ingredients -except the tomatoes-into a blender and blend to a purée; do this in batches if necessary. Pour the puree into a heavy-base saucepan and gently bring to a simmer over low heat. Cook, stirring occasionally, for about 40 min. Be careful that the mixture does not stick to the bottom of the pan and burn. You are aiming for a jam-like consistency at this stage.
Bring a large pot of water to the boil. Cut a small cross into the base of the tomatoes with a paring knife and plunge them into the boiling water. Leave for 30–60 seconds, then remove and plunge into iced water to cool.
Drain the tomatoes then skin, deseed and chop into rough chunks. Add to the saucepan and stir through. Cook over a low heat for a further 3 hours, stirring from time to time. The oil will separate to the surface and the colour of the mixture will turn a deep reddish brown.
Remove from the heat and fill into hot, sterilised glass jars.
Store in the refrigerator for up to 3 months – it improves with age.