In a medium size pot heat up 2 tablespoons of oil and cook the onions gently for about 3 minutes until softened (but not browned) stirring frequently. Add the spiralized carrot, celeriac and potato and continue cooking over a low/medium heat for 4 minutes stirring often.
Add the spiralized zucchini, stock and pepper to taste. Cover and bring to the boil then lower the heat and simmer for 3 minutes stirring occasionally.
Remove from the heat, add the chopped herbs, adjust the seasoning and serve.
Prepare the vegetables just before making the soup (peeled potato and celeriac become darker after a few minutes).
Add the zucchini a little later - it needs less cooking time than the other vegetables (see Instructions for details). That's why I also recommend spiralizing it separately.
Add hot vegetable stock to shorten cooking time even further.
Do not overcook the soup as spiralized vegetables cook very quickly. They should retain some texture.
Other vegetables you can add: sweet potato, butternut squash, turnip, parsnip, marrow etc.