Heat your oven to 200°C/Fan 180°C/Gas 6. Carve the skin off the celeriac and chop into small chunks. Cut the base from the celery and pick off any leaves. Set them aside. Cut the celery into thick slices. Peel and thickly slice the onion.
Put the celeriac and celery onto a lined baking tray. Drop on the unpeeled garlic clove. Scatter over the fennel seeds. Drizzle over 1 tbsp oil and season. Roast for 20 mins.
Pluck the garlic clove from the tray. Set aside. Toss the veg together and return to the oven for a further 20 mins.
When the veg are nearly cooked, heat a heavy based pan. Pour in 1 tbsp oil. Fry half the sage leaves for 1-2 mins till crisp. Remove from the pan using a slotted spoon.
Scrape the veg into the sage oil. Stir. Pour in 600ml boiling water. Add the uncooked sage leaves. Cover. Simmer for 5 mins.
Blitz the soup with the fresh celery leaves. Add more water if needed. Taste and season. Serve in warm bowls with the reserved crispy sage leaves.