Melt butter in a large saucepan over medium-high heat, about 1 minute. Add leek, celery and smoked paprika; cook, stirring occasionally, until the vegetables are slightly tender, about 5 minutes. Stir in tomato paste; cook, stirring, for 1 minute. Add flour; cook, stirring constantly, for 1 minute. Gradually add broth (or stock) and sherry, stirring constantly and scraping the bottom of the pan to release any browned bits.