cut off both ends of the leeks (you don't use the leafy dark green parts)
half leeks lengthwise and thinly slice
use a colander or strainer to wash leeks (soil and grit can get in between the layers of the leeks and I find this method works the best to wash it away)
melt 3 tbsp butter in saucepan over medium heat
add leeks
cook for about 5 minutes or until leeks soften
add cream and chicken broth
turn temperature up to medium-high
I like to use my immersion blender here. You can do this in a regular blender or a food processor or skip this step all together.
add dill, mustard powder, salt and pepper (to taste)
Boil for about 10 minutes or until sauce thickens