Preheat the oven to 180°C.
Cut the beets and onion into wedges (don’t peel them) and put them in a baking tray. Tuck the garlic between them. Scatter the herbs on top, drizzle with oil and vinegar, then cover the tray with foil.
Bake in the oven for 30 minutes or so, until the veggies are soft.
When the veggies have cooled down a bit, remove the skins and rough parts. (You can chill them, if you want to make the soup later).
Put the beet, onion and garlic pieces into a saucepan, including any cooking jus. Add the stock and bring to the boil. Simmer for a few minutes, then turn off the heat. Add the basil and, using a stick blender, blitz the broth into a thick, smooth, velvety soup. Season with salt and pepper, if needed.
Serve with a spoonful of balsamic vinegar.