Clean mussels: Scrape off barnacles, remove beard fibers, and scrub with a stiff brush under cold water.
Soak cleaned mussels in lightly salted cold water for 2 hours (or longer, adding ice if needed).
Boil mussels until opened.
Fry chopped onion in olive oil. Add garlic and parsnip, and lovage. Simmer with white wine.
Strain mussel broth and add to stewed vegetables. Stir in sour cream.
Add peeled mussel meat to the soup. Add 250-300ml water and Parmesan cheese. Simmer for 10 minutes.
Prepare a roux with butter and flour. Add to the soup.
Serve with a lemon wedge.