Cut the gammon, carrot, celeriac and tomato into bite-sized chunks. Finely slice the onion.
Heat the oil, then add the gammon and onion. Cook for a few minutes on a medium heat until the onion has softened and the gammon is cooked through.
Add the paprika, cornflour and garlic, then stir gently for one minute.
Add the beans and enough water to cover well, then add the bay leaf and the chopped vegetables. Add more water, if needed. Bring to the boil.
Simmer over low heat under cover. Cook for about 15-20 minutes until the vegetables are tender.
Add a spoonful of vinegar or soured cream just before serving.