Heat the oil in a large saucepan and add the onions, garlic and celery. Cook, covered, on a lowish heat for a good 15 minutes, until the onions and celery are soft. Stir through the cinnamon.
Add the beetroot and pears to the saucepan and continue to cook, covered, for 10 minutes, removing the lid to stir halfway through. Pour over the stock, cover again and simmer for 40 minutes or until the beetroot is tender and cuts like butter.
Use a hand-held blender or food processor to purée the soup, in batches if necessary, until smooth. Return the soup to the saucepan and warm through. Season well with salt and black pepper.
Heat a griddle or frying pan over a high heat and grill the pancetta until crispy. Remove from the pan and quickly do the same to the brioche, which won’t take a second to toast on a hot griddle.
Serve the soup in bowls, each garnished with two grilled strips of pancetta. Put toasted brioche on the table.