Preheat the oven to 190°C.
Cut the beetroots and onion in half (both with skin on). Cut the belle pepper into 4 chunks.
Peel the celeriac & swede the cut them into 3 cm chunks.
Scatter all these veggies in a large roasting tray. Add the garlic ( with skin on) and chilli if used. Drizzle with oil then scatter the oregano on top. Toss well, then bake in oven for approx. 30 minutes until soft.
When the veggies are tender take them out of the oven and let them cool a bit. Remove the skin of the onion, garlic, pepper and the stem of the chilli. Gently peel off the skin of the beetroot (if organic, keep it on).
Put the baked veggies in a large saucepan and add the stock. Bring to the boil and cook for 5-10 minutes. Add the spinach and chopped herbs of your choice, put the lid on and turn off the heat. Rest for 5 minutes, then process the soup with a hand-held blender until you get a smooth, velvety texture. Add some cider vinegar or lemon juice and serve.
You can serve it with a dollop of yoghurt/crème fraîche/ sour cream, if dairy is not an issue for you.