Fry the leeks in the butter in a saucepan until softened, add celeriac, potato and thyme.
Pour in the broth and let simmer with the lid on for 1 ½ hours, until the celery is very soft. Dilute with more liquid if it feels like it evaporates too much. Use a ballongvisp or masher and pound the celery in large pieces and let the broth to be soaked up.
season to Taste with salt and top with sunflower seeds, gruyère, 2 tablespoons of canola oil and chives when serving.