Trim the fennel. Cut the fennel and onion into wedges (don’t peel the onion) and put them in a baking tray. Add the garlic cloves. Scatter the thyme leaves all over then drizzle the oil on top.
Wrap the beetroots in foil (don’t peel them) and place the parcel next to the fennel.
Bake in medium hot oven (about 170-190°C) for 30 minutes or so until the fennel is soft. Remove the beetroot, then carefully open the parcel. Using a skewer, check if they are cooked through. If not, put them back (wrapped in foil) in the oven for a further 15 minutes or so.
Transfer the fennel and onion/garlic (their skins discarded) into a pan or into a jar for later use. When the beetroot is cooked through, remove the foil and let them cool. When cooled enough, remove the skin with a knife. Cut the flesh into chunks and add to the pan or jar.
When you’re ready to make the soup, add the stock and herbs, bring to the boil, then whizz with a stick-blender until you get a creamy soup. Add more stock if it’s too thick.
Serve with some chopped herbs and a dash of apple cider vinegar or a dollop of Greek yogurt/skyr.