Photograph: Dan Jones
Preheat the oven to 200°C, fan 180°C, gas 6. Peel and deseed the squash; cut three-quarters into chunks and season. Roast for 35-40 minutes in a tin with half the oil. Dice the remaining squash.
Soften the shallots and celery in the butter and remaining oil. Purée the roast squash in a blender with half the stock; add to the shallots and celery with all the stock and diced squash; simmer for 15 minutes.
Combine the dumpling ingredients. In a frying pan, fry spoonfuls of the mixture in butter and oil over a low-medium heat for 2-3 minutes on each side. Fry the whole sage leaves in butter.
Serve the soup topped with the dumplings and frizzled sage leaves.
Get ahead: make to the end of step 2 up to 2 days ahead; you can freeze the soup at this point, too.