Brush the mussels thoroughly under running water. Sort out mussels that are already open. Place the mussels in a large pot and pour on the white wine. Bring to the boil, remove from the heat and leave to stand for 5 minutes in a closed pot.
Remove the mussels from the pot and sort out unopened mussels. Let the mussels cool down a little. Pour the mussel stock through a fine sieve and collect. Peel onion and garlic. Chop the onion finely.
Cut garlic into wafer-thin slices. Wash parsley and dab dry. Pluck the leaves from the stalks, except for a few for garnishing, and chop finely. Heat the oil in a pot. Sauté onion, garlic and parsley in it.
Deglaze with wine-shellfish stock and let it boil down for 2-3 minutes. Cook pasta in boiling salted water according to package instructions. In the meantime, remove half of the mussel flesh from the shells and warm it up with the remaining mussels in the stock.
Season to taste with salt, pepper and lemon juice. Drain the pasta and mix with the stock. Serve garnished with parsley.