Sauté red onion and celery with evoo in a soup pot, for about 3 minutes. Add garlic and saute for another minute.
Add mussels, dill, and chili flakes to soup pot. Put on lid and let steam for 10 minutes. If too much water, take off lid for another minute or two to let some evaporate out.
Serve with a side salad, baked fries for dipping, or crunchy garlic bread to soak up the broth.