SERVES 4

Elegant and surprisingly easy to prepare, the salmon in this dish is immersed in a buttery, wine–and–mussel-infused broth. (Nage is the French word for swim.)

6 stalks asparagus (about 3 oz.)
9 tbsp. unsalted butter
1 large shallot, finely chopped
4  6-oz. skinless boneless salmon filets
Kosher salt and freshly ground black
   pepper, to taste
24 mussels, scrubbed and debearded
1 1⁄2 cups white wine
1⁄2 cup fresh or frozen peas
1 tbsp. finely chopped fresh tarragon
1 tbsp. finely chopped fresh chives
1 tbsp. finely chopped flat-leaf parsley
1⁄2 tbsp. finely chopped fresh dill

1. Heat oven to 225°. Snap off and discard ends from asparagus. Thinly slice stalks on the bias; leave tips intact. Set aside. Grease a 10" straight-sided skillet with 1 tbsp. butter. Sprinkle skillet with shallots. Season filets with salt and pepper; arrange in skillet. Scatter mussels around filets; pour in wine with 1 1⁄2 cups water. Boil, and reduce heat to medium-low; simmer, covered, until mussels open, 2–3 minutes. Remove from heat; set aside, covered, to let steam, until fish is just cooked through, 3–4 minutes. Using a spatula, transfer fish to a baking sheet; transfer mussels with a slotted spoon to sheet, leaving broth in skillet. Keep fish and mussels warm in oven.

2. Place skillet over high heat; bring broth to a boil. Whisk in remaining butter, 1 tbsp. at a time, until smooth. Add asparagus and peas; cook until tender, 2–3 minutes. Remove from heat; stir in tarragon, chives, parsley, and dill. Season with salt and pepper. Divide fish and mussels among 4 bowls; divide broth between them.