In a large stock pot or shellfish pot, heat Olive Oil (2 tablespoon), add Onion (1/2), Celery (1 stalk), Garlic (1 clove) and the bottom part of the Leek (1) and sweat for 5-7 minutes (without colouring).
Add Belgian Ale (330 milliliter), and Ground Black Pepper (to taste), increase heat and bring to a boil for 5 minutes.
Add Fresh Mussels (1 kilogram). Cover, and remain on a medium-high heat, and allow mussels to steam until they open, about 5 minutes (discard any unopened ones).
Remove mussels (or strain remembering to keep cooking liquid).
Add Butter (50 gram) to pot, and cook for about 2-3 minutes.
Either return mussels to the pot and cover to heat through, or pour sauce over mussels in serving bowls.
Garnish with fresh chopped Fresh Parsley (to taste). Serve immediately, with fries and homemade mayo. Enjoy!