How to Make It
Heat a 12-inch, wide-bottomed, high-sided sauté pan over medium, and add 2 tablespoons butter. When it foams, add shallots, and cook, stirring occasionally, until tender, 3 to 4 minutes. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add garlic, and cook, stirring often, until fragrant, about 1 minute. Add mussels, wine, and bay leaves, and cover tightly. Steam until mussels open halfway, about 4 minutes. Stir mussels, and add cream. Cover and steam until mussels have opened fully, about 1 minute. (Discard any mussels that do not open.)
Whisk together egg, egg yolks, and 6 tablespoons lemon juice in a small bowl. With pan over medium, add egg mixture to mussel pan liquid in a steady stream, stirring constantly to coat the mussels in a lightly thickened sauce. Remove from heat and stir in remaining 1 tablespoon butter, remaining 3/4 teaspoon salt, remaining 1/4 teaspoon pepper, and, if desired, 1 tablespoon lemon juice. (The acidity of the sauce should verge on bracing.) Add dill and parsley, and serve immediately right from the pan or over a pile of white rice to sop up the juices.