1. Sauté the aromatics: Melt the butter in a deep, large saucepan over medium-high heat. Add the chopped onions and fry for 5-10 minutes until they are softened and starting to colour.
2. Create the spice base: Add the crushed garlic and cook for another minute. Sprinkle in the plain flour and curry powder, stirring continuously for 1-2 minutes to cook out the flour taste.
3. Simmer the soup: Gradually stir in the stock, ensuring there are no lumps, and bring the mixture to a boil. Add the cubed celeriac, cover the pan with a lid, and reduce the heat. Simmer gently for 20-30 minutes, or until the celeriac is completely tender when tested with a knife.
4. Blend until smooth: Remove the pan from the heat. Strain the solids from the liquid, reserving the cooking liquor in a bowl. Transfer the boiled vegetables to a food processor or blender and whiz until completely smooth.
5. Finish and season: Return the smooth vegetable purée and the reserved cooking liquid to the saucepan. Whisk to combine thoroughly. Bring the soup back to a boil and season generously to taste with salt and pepper.

Serve: Serve piping hot into bowls. For an extra touch of luxury, finish with a swirl of fresh cream and a sprinkle of chopped chives if you like.