To clean mussels, put them in a large bowl of cold water with a handful of flour of 30 minutes. They will drink the water and disgorge any sand.
Source: French Mussel Bisque, Barefoot Contessa How Easy Is That?
1 (750-mL) bottle dry white wine, divided
3 pounds fresh mussels, cleaned
6 tablespoons (¾ stick) unsalted butter
1 ½ cups chopped yellow onion (2 onions)
1 large leek, white and light green parts, cleaned and chopped
2 carrots, chopped
4 teaspoons minced garlic (4 cloves)
½ teaspoon saffron threads
Kosher salt and freshly ground black pepper
4 whole canned plum tomatoes, chopped
1 ½ cups half-and-half
1 cup heavy cream
2 tablespoons minced fresh parsley or dill
Bring 1½ cups water and 1 cup of the wine to a boil in a large, shallow pot. Add the mussels, cover the pot, and cook over medium heat for 5 minutes, until the mussels open. When the mussels are cool enough to handle, separate the mussels from the shells and set aside, discarding the shells and any mussels that didn’t open. Strain the cooking liquid through a cheesecloth-lined sieve into a bowl and reserve.
Meanwhile, melt the butter in another large heavy-bottomed pot over medium heat. Add the onions, leek, carrots, garlic, saffron, 2 teaspoons salt, and 1 teaspoon pepper and sauté, stirring occasionally, for 5 minutes. Reduce the heat to low, cover, and cook for 20 minutes. Add the tomatoes and cook for another minute.
Add 2 cups of the reserved cooking liquid (discard the rest) and the rest of the wine, bring to a boil, lower the heat, and simmer uncovered for 5 minutes. Add the reserved mussels, the half-and-half, and the cream. Heat until just heated through but not boiling. Stir in the parsley or dill, 1 teaspoon salt, and ½ teaspoon pepper. Serve hot.