Step 1

In a large, deep skillet, heat the olive oil. Add the onion and garlic and cook over moderate heat, stirring occasionally, until softened, about 10 minutes. Add the thyme sprigs, wine and Pernod and bring to a boil. Stir in the mussels, cover and cook over moderate heat until they open, about 3 minutes. Transfer the mussels to a bowl, discarding any that do not open.

Step 2

Boil the liquid in the skillet over moderately high heat until reduced by half, about 2 minutes. Stir in the cream and simmer just until the sauce is thick enough to coat the back of a spoon, about 3 minutes. Discard the thyme sprigs and stir in the mussels along with any accumulated juices. Season with kosher salt and pepper.

Step 3

In a large pot of boiling salted water, cook the tagliatelle until al dente; drain well. Add the pasta and the chopped tarragon to the skillet; toss to coat thoroughly with the sauce. Transfer the pasta and mussels to a large bowl. Garnish with the tarragon leaves and the black and white salts and serve.