Cut the onion in half, peel it and cut it into a 5mm (¼") dice.
Cut the celery sticks into the same size dice as the onion.
Peel and slice the garlic cloves as thinly as you can.
Peel and slice the Jerusalem artichokes into 5mm (¼") thick coins. Place them in a bowl of cold water with a splash of lemon juice to prevent them from going brown.
Heat the butter over a medium heat in a 20cm (8") saucepan. Add in the onion and celery and cook for 5 minutes.
Once the onions are soft and translucent, add in the garlic, black pepper, and cumin seeds, then cook for 1 minute.
Dry the Jerusalem artichokes on some kitchen paper and add them to the pan, stir and cook for 5 minutes.
Pour in the vegetable stock and the lemon juice, add a lid, turn the heat down to a gentle simmer, then cook until the artichokes are soft, about 15-20 minutes. Have a taste of the stock and add more salt now if required.
Then blend the soup until smooth. I like to pass the soup through a fine mesh sieve back into the pan.
Stir through the sour cream and bring the soup back to temperature, taking care not to boil it hard.
If you want to add the Jerusalem artichoke crisp topping, heat 2-3mm (⅛") of oil in the base of a 24cm (10") pan over a high heat,When the oil is almost hot, slice the artichokes as thin as you can. I use a mandolin set at 1mm, then toss them into the oil and fry until golden. Drain on kitchen paper.Then serve.
Video
Notes
You may need to add a little extra stock when blending to get a texture that you like.