Preheat your oven to 200°C/Fan 180°C/Gas 6. Trim the top off the aubergine and chop into small bite-size chunks. Chop the tomatoes into wedges. Scatter both into a large roasting tin and toss with 1 tbsp oil and a pinch of salt and pepper. Slide into the oven and roast for 25 mins, till the veg are caramelised and tender.
Peel and thinly slice the onion. Peel the garlic cloves and crush or finely chop them.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onion and garlic. Sprinkle in 1 tsp ground coriander along with a little salt and pepper. Fry for 3-4 mins, stirring occasionally, till the veg start to soften.
Pour the buckwheat into the pan and crumble in the vegetable stock cube. Cover with 800ml boiling water and stir well to combine. Return the pan to the boil, then reduce to a simmer and cook for 10 mins.
Scoop the roasted tomatoes and aubergines into the pan, and use a stick blender to whizz till smooth. No stick blender? Transfer the soup into a blender (in batches if necessary) and blitz. Have a taste of the soup and add salt and pepper if needed. Ladle the soup into bowls and serve.