In a large heavy-based saucepan, cook the garlic, onions and thyme in the butter over a medium heat until softened, adding a little stock if necessary to prevent sticking. This should take about 7 minutes. Reduce the heat, stir in the corn flour and cook until lightly brown. Add the stock gradually, stirring to avoid lumps. Increase the heat, bring to the boil and simmer for 30 minutes. In small batches, pour the soup into a blender and blend until smooth. Return to the heat. Drain and rinse the beans; add beans to the soup with the vinegar.
Ready in 50 min