1. Make a roux by melting the butter in a skillet. When melted, add 3 tablespoons of flour little by little and stir into a thick paste. Reserve remaining flour.
2. Slowly stir in the broth. Stir until the roux absorbs the liquid.
3. Simmer for a couple of minutes on a low heat while you stir in the onion, sage and the diced turkey. The mixture should thicken and turn into a heavy, thick sauce.
4. Pour the mixture into a shallow container. Cover and refrigerate for several hours.
5. When mixture is solid, take a heaping tablespoon and quickly roll it into a small ball.
6. Roll lightly in reserved flour, then the egg, then breadcrumbs.
7. When done, refrigerate the bitterballen for 1 hour.
8. Heat oil to 375 degrees.
9. Fry 4 or 5 bitterballen at a time, until golden. Drain on paper towel.
10. Serve with mustard or gravy for dipping.