Sweat the onion in the butter until soft and translucent, but don’t allow it to brown. Stir in the flour and cook to a roux for 2 minutes.
Pour in the milk and simmer, stirring occasionally, for 20–30 minutes, when it should be thick and creamy and reduced to about
Purée in a food processor or blender. Pour back into a clean saucepan, taste and adjust the seasoning if necessary. You can add a few tablespoons of cream if you like.
Reheat gently before serving.