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1
Melt 60 g of butter in a heavy-based saucepan over medium-low heat. Add the sliced carrots, onion, and a pinch of salt. Sweat, stirring occasionally, for 15–20 minutes. This step decides the dish: the carrots must soften entirely and deepen in colour to a richer orange as their sugars concentrate — liquid added before this transformation is complete will dilute rather than develop their flavour.
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2
Add the diced potato, 1 litre of chicken stock, the bouquet garni, and a pinch of sugar if the carrots lack sweetness. Bring to a gentle simmer and cook for 25–30 minutes. The soup is ready to purée when every piece of carrot and potato offers no resistance to a knife.
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3
Remove and discard the bouquet garni. Pass the soup through a moulin-légumes or blend in a blender until smooth. Unlike potato, carrot fibres require thorough breakdown; blend fully until no fibrous texture remains. The purée should feel completely uniform on the back of a spoon.
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4
Pass the purée through a fine sieve, pressing firmly. The result should be perfectly smooth and silky — any graininess indicates incomplete sieving.
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5
Return the soup to gentle heat. Adjust consistency with additional stock to the desired body. Stir in the 80 ml of double cream, then remove from heat and whisk in 30 g of cold butter. Season with salt, white pepper, and a small grating of nutmeg. The finished soup should be the colour of autumn — a warm, deep orange, neither pale nor brown.
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6
Fry the bread cubes in butter over medium heat, turning, until golden and crisp on all sides. They should rattle lightly against the pan when shaken — the sign that all moisture has left and the crust is set.
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7
Ladle the soup into warmed bowls. Scatter the buttered croutons at the centre and finish with a few picked chervil leaves.