How to Make It
In a large saucepan, combine the mussels, wine, dill sprigs and 2 tablespoons of the Lillet and bring to a boil. Cover and steam until the mussels open, about 4 minutes. Transfer the mussels to a bowl and strain the juices into a medium bowl. Remove the mussels from their shells; discard any mussels that don't open. Rinse out the saucepan.
Return the juices to the saucepan and simmer until reduced to 3/4 cup, about 5 minutes. Stir in the crème fraîche, shallot and the lemon and grapefruit zests.
Meanwhile, in a large pot of salted boiling water, cook the pasta until nearly al dente; drain. Add the pasta to the saucepan along with the remaining 2 tablespoons of Lillet. Cook over high heat, stirring frequently, until the sauce is thick and creamy, about 4 minutes. Stir in the mussels and chopped dill and season the pasta with pepper. Serve right away.