Heat a couple of tablespoons of water in a large pan and add the onion and celery. Pop on the lid and sweat on a medium/low heat for 5 minutes.
Whilst this is cooking, peel the celeriac and apple and chop into small chunks - both start to oxidise when open to the air, so you want to work fast so they don't get too discoloured.
Add them to the pan with the garlic and continue to sweat with the lid on for a few minutes on a low heat to caramelise a little.
Add the thyme, celery salt and stock to cover. Pop the lid back on and simmer for 20 minutes or so until all the veggies and apple are soft.
Leave to cool for a few minutes, then blitz with a hand blender until smooth. If the soup is too thick for you, add more stock or dairy-free milk to make it creamer.
Check the flavour and season with salt and pepper as needed.
Reheat gently and serve, garnish with a glug of oil and chives if you so desire.