1 tbsp olive oil
1/4 large celeriac (about 250g, untrimmed), peeled and cubed
3 large fennel bulbs (about 750g), trimmed and sliced (feathery fronds reserved)
Finely grated zest of 1 orange
500ml vegetable stock
Salt and pepper
4-6 tbsp crème fraîche
Heat butter and oil in a large saucepan over a medium heat. Add shallots and sweat gently for a few minutes. Add the celeriac and fennel, stir well, then cover for about 10 minutes.
Add the orange zest, stock and season. Bring to the boil, then simmer for about 15 minutes until veg is tender.
Purée the soup in a blender until completely smooth, adding a touch more stock to loosen the consistency if necessary.
Reheat the soup if necessary, check seasoning, serve with crème fraîche and fennel fronds.