1⁄8 tsp. crushed saffron threads (about 12 threads)
9 tbsp. unsalted butter
1⁄2 cup minced fennel bulb, fronds reserved
1 large shallot, minced
4 (6-oz.) skinless, boneless Norwegian salmon filets
Kosher salt and freshly ground black pepper, to taste
1 cup white wine
3 tbsp. dry white vermouth
1 tbsp. minced fresh chives
1 tbsp. minced fresh flat-leaf parsley leaves
1 tbsp. minced fresh tarragon leaves
Heat oven to 225. Combine saffron and 1 1⁄2 cups warm water in a small bowl; set aside. Grease a 10″ straight-sided skillet with 1 tbsp. butter. Arrange fennel and shallots in bottom of skillet. Season salmon with salt and pepper; arrange in skillet. Scatter mussels around filets; pour the reserved saffron liquid and the wine around the salmon.
Cover and bring to a boil, reduce heat to medium-low, and cook until the mussels open, about 3 minutes. Remove pan from heat; set aside, covered, to let steam until salmon is cooked through, about 3 more minutes. Using a metal spatula, transfer fish and mussels to a baking sheet, and transfer baking sheet to oven.
Return skillet to high heat; add vermouth and bring to a boil. Whisk in remaining butter 1 tbsp. at a time. Remove pan from heat and stir in chives, parsley, and tarragon. Season broth with salt. Divide fish and mussels between 4 bowls and spoon broth over top. Garnish fish with reserved fennel fronds.