Half-fill a preserving pan with water, bring to the boil, then add the runner beans. Return to the boil and cook for 3 minutes. Drain into a colander, refresh with cold water, then drain again. Thinly slice the beans or roughly chop in a food processor.
Add the vinegar and sugar to the drained preserving pan, then add the onions. Cover and bring to the boil, then reduce the heat and simmer for 10 minutes.
Mix the remaining dry ingredients together in a bowl, then stir in the measurement water until smooth. Stir this into the vinegar mixture, then simmer, uncovered, for 10 minutes, stirring until smooth and thickened.
Stir the blanched beans into the vinegar mixture and cook gently for 10 minutes, stirring frequently until just tender. Ladle into warm, dry jars, pressing the beans down well in the vinegar mix. Disperse any air pockets with a skewer or small knife and cover with screw-top lids. Label and leave to mature in a cool, dark place for at least 3 weeks.