Heat the olive oil or butter in a large pot over medium heat. Add the diced onion and cook for 4–5 minutes until soft and translucent.
Add the minced garlic and cook for about 30 seconds until fragrant.
Stir in the chopped carrots and diced potato. Cook for 2–3 minutes, stirring occasionally.
Pour in the vegetable stock and add the dried thyme. Bring everything to a boil, then reduce the heat and simmer for about 15 minutes, or until the carrots and potatoes are tender.
Add the chopped cooked beetroot and simmer for another 5 minutes.
Using an immersion blender, blend the soup until smooth and creamy. Alternatively, carefully transfer the soup to a blender in batches.
Stir in the cooking cream (or plain yoghurt) and add the lemon juice if using. Taste and season with salt and freshly ground black pepper.
Serve hot with your favourite toppings and plenty of crusty bread.