To make the white sauce
in a saucepan heat up the butter over a low heat until melted. Add the flour and quickly whisk until the mixture thickens and the flour is no longer visible. Then add a bit of milk, increase the heat and whisk until the sauce thickens. Add the rest of the milk and continue whisking until the sauce thickens and starts bubbling up. Remove from the heat, season to taste and add the dill. Stir and set aside.