Melt the butter in a pan.

Add the onion and cook over low heat, stirring occasionally, until softened and translucent but not browned.

Whisk in the flour

Pòur in all the milk, whisking constantly until it starts to boil.

Season with salt, lower the heat, cover and simmer gently, stirring occasionally, for at least 20 minutes.

A few minutes before removing the pan from the heat, stir in the paprika