Preheat the oven to gas 6, 200ºC, fan 180ºC. In a large saucepan, soften the onion in 1tbsp olive oil for 5 minutes. Shred 3 of the sage leaves and add to the onion. Add the chestnuts, carrots and stock to the pan and bring to the boil. Reduce the heat, partially cover and simmer for 25 minutes, until the carrots are tender.
Meanwhile, roll out the puff pastry on a lightly floured surface to form a rectangle just more than 35x12cm (14x5in). Trim the edges to neaten, then cut into 3 smaller rectangles and halve each diagonally to make 6 triangles. Scatter evenly with the Gruyère and press down firmly. Roll up from the wider 'base' of each triangle to make 6 'cigars'. Bake on a lined baking sheet for 15 minutes or until golden. Cool on wire racks.
Heat 1tbsp oil in a small frying pan and add the rest of the sage leaves. Fry until crisp and fragrant but not coloured, then drain on kitchen towel. Use a handheld blender to purée the soup until smooth (or pour into a blender, purée, then return to the pan). Season to taste.
Reheat gently until piping hot, then ladle into warmed bowls. Top with a spoonful of crème fraîche, the sage leaves and pepper. Serve Gruyere sticks on the side.
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Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.