Cook the pasta as you normally would but in boiling mussel broth instead of water. How much salt you need to add depends on how well-seasoned the broth is.
When the pasta is halfway done, heat the olive oil, add the garlic and chili flakes and cook over medium heat until the garlic bits are lightly browned.
Drain the pasta and add to the oil and garlic. Pour in a couple of tablespoonfuls of the mussel broth. Add the mussels on the half shell. Toss. Taste and add salt and pepper, as needed. Once the mussels are heated through, turn off the heat.
Add the parsley, toss until the parsley is well distributed.
Serve the fettuccine aglio e olio with mussels immediately.