Preheat the oven to 200°C/Fan 180°C/Gas 6. Line a baking tray with greaseproof paper and roast any suitable vegetables, such as radishes and tomatoes, for 20 minutes.
Make the dressing by combining the ingredients in a food processor and blending until smooth, adding water until you reach your desired consistency.
Heat a saucepan with a drizzle of olive oil and toast the buckwheat, stirring constantly, for 2 minutes. Cover with water and add a pinch of salt, then bring to the boil. Let the buckwheat simmer for 10 minutes, or until tender, then drain and spread it out on a clean tea towel to dry for 10 minutes.