Pre-heat the oven to 180C.
Peel and slice the onion. Peel the carrots and celeriac. Chop into smallish dice. Leave the skin on the garlic cloves
Pop the prepared veggies into a large roasting dish, making sure the garlic cloves are nestled on top in the centre. Spray a little olive oil over the top if you wish.
Pop the tin in the centre of the oven and roast for 10 minutes.
Using a large spoon, stir all the veg up and pop back in the oven for another 10 minutes or until they're soft to the touch.
Remove the roasting tin from the oven and leave to cool for a couple of minutes whilst you make the vegetable stock.
Once cool enough to handle, peel the garlic cloves and discard the skins. Tip the roasted veg into a large blender jug and add the celery salt and black pepper. Add the beans now if using. Pour in 600mls of the stock and blend to smooth.
Add more stock if the soup is too thick. Check the flavour and add more seasoning if desired. Blend again.
Serve immediately whilst still hot. If the soup has cooled down too much, pour into a pan and reheat gently.
If you are cooking the soup in a pan, heat 2 tablespoons of water in the base of a large saucepan and add the onion, carrot and celeriac. Saute for 10 minutes until soft. Peel and chop the garlic and add to the pan and cook for another couple of minutes. Pour over the veg stock, add the celery salt and simmer for 10 minutes until everything is cooked. Add beans now if using. Leave to cool a little then blend with a stick blender. Season with black pepper and add more celery salt if needed.
