Heat the butter and oil in a saucepan, sauté the onion, then let the flour turn light brown. Stir constantly. Pour part of the broth and add the cream. Pour the broth up depending on the consistency and taste. Stir in the mustard, season with white pepper.
As a caper fan, I add the entire contents (drained) of the caper glass to the sauce.
Serve with jacket potatoes and cold, hard-boiled eggs.