Melt the butter in a large saucepan over a low heat, add the onion and slivered almonds and cook gently for 3–4 minutes until the onion is softened and the nuts are golden. Stir in the Brussels sprouts.
Pour the stock into the saucepan, bring to the boil then add the bouquet garni. Reduce the heat to low, cover and simmer gently for 10–15 minutes until the sprouts are tender when tested with the point of a knife.
Using a slotted spoon, discard the bouquet garni, then lift out the sprouts, onion and nuts and transfer them to a blender or food processor. Add a little of the cooking liquid to the sprouts then whizz to a smooth purée. Return to the saucepan, stir well and season to taste with nutmeg and freshly ground black pepper.
Stir in the cream, if you are using it, and warm through without boiling. Serve garnished with a sprinkling of chives or toasted flaked almonds.