Squeeze excess liquids out of the grated courgettes and fry for 3 minutes in a non-stick pan until most of liquid has evaporated. Add oil and garlic and cook and stir for about 2 minutes until fragrant. Stir in flour and cook for another 2 to 3 minutes. Take off the heat and allow to cool for 10 minutes.
In a medium sized bowl combine yoghurt and walnuts. Mix with cooked courgettes and add salt, pepper and vinegar to taste. Serve immediately or keep refrigerated until serving.