1. To make the soup, melt 25g butter in a medium saucepan and gently cook the onion for 2-3 minutes until softened but not browned. Add the potato and stir to coat with the buttery onion. Pour in the stock, bring to the boil then simmer for 12-15 minutes or until the potato is tender.
2. Add the watercress (reserving 1-2 tbsp to garnish) and simmer for a further minute. Remove from the heat and leave the soup to cool slightly then blend or whizz in a hand-blender to make a smooth purée. Stir in the buttermilk (reserving 1-2 tbsp to garnish the soup) then cool and chill until ready to serve.
3. Season the crabmeat with the nutmeg and lemon zest plus lemon juice to taste.
4. Lightly toast the 4 pieces of sourdough, spread with the remaining butter and top with the seasoned crab. Pour the chilled soup into four bowls, add a swirl of buttermilk and the reserved watercress leaves. Serve with the crab toast.