Peel and coarsely chop the ginger and garlic. Clean the chilli, cut lengthwise, remove seeds, wash and chop coarsely. Wash and roughly dice the tomatoes. Puree them finely in a tall mixing bowl with a hand blender.
Bring tomato puree, sugar, vinegar and fish sauce to the boil and simmer for about 30 minutes, stirring occasionally, until the jam is reduced to about half.
Pour the jam immediately into jars. Tastes great with grilled food or even pasta.