Heat your oven to 180°C/Fan 160°C/Gas 5. Split the bulb of garlic into cloves (leave the skins on at this stage) and scatter into a roasting tin. Pour over 1 tbsp oil and sprinkle in a pinch of salt and pepper. Toss to coat in the oil and slide into the oven. Roast for 18-20 mins, till softened and caramelised.
Meanwhile, peel the onions and finely chop them. Pick the rosemary leaves off their stalks and finely chop them.
Pour 1 tbsp oil into a pan and warm over a medium heat. Slide in the onion and rosemary. Sprinkle in a pinch of salt and pepper and fry for 5 mins, stirring occasionally, till slightly softened.
Stir the buckwheat into the onions and crumble in the stock cube. Pour in 750ml boiling water and stir well to combine. Cover the pan with a lid and return to the boil. Reduce to a simmer and cook for 8-10 mins, till the grains are tender.
When the garlic is cooked, leave it to cool slightly, then squeeze the cloves out of the skins. Add to the pan, along with the spinach. Stir till the leaves have wilted.
Use a stick blender to whizz into a smooth soup. No stick blender? Transfer the soup into a blender (in batches if necessary), and blitz. Ladle the soup into warm bowls and serve.