- Heat a large saucepan over medium-high heat, add olive oil when hot.
- Add the leeks, celery, shallots, salt and pepper to taste and sauté until the shallots are soft, about 2 minutes.
- Add the mussels and American pale ale cover and steam for about 3 minutes or until the mussels open. Discard any mussels that do not open.
- Remove the opened mussels from the broth. Whisk in the butter and Dijon mustard.
- Pour the mussels into a serving bowl, spoon the liquid back over the mussels and sprinkle with parsley.