While it’s simmering, prepare the roux: melt the butter in a saucepan over medium heat, then add the flour and mix well with a rubber spatula.
When the mixture is thick and paste-like, gradually add the remaining broth, stirring constantly.
The mixture will thicken as it cooks. If it thickens too much and you run out of broth, you can add water or take a little from the sauté pan.