3 tablespoons whole-grain mustard
1 ½ tablespoons honey
2 teaspoons finely chopped fresh rosemary
1 teaspoon toasted whole fennel seeds, crushed in a mortar and pestle, optional
4 6-oz. salmon fillets, skin on (or Arctic char)
Kosher salt and freshly ground black pepper
3 tablespoon unsalted butter or margarine
3 large leeks, white and light green parts only, thinly sliced
1/2 cup dry white wine
Position a rack in the center of the oven and heat the oven to 400°F. Line a small rimmed baking sheet with parchment.
In a small bowl, combine the mustard, honey, rosemary, and 1/2 tsp. of the fennel seeds. Place the fish skin side down on the prepared baking sheet, season with salt and pepper, and spread with the mustard mixture. Roast until the fish is just opaque in the center (check by poking into one piece with a paring knife), about 10 minutes.
Meanwhile, in a 12-inch skillet, melt 2 tablespoons of the butter/margarine over medium heat. Add the remaining 1/2 tsp. fennel seeds, leeks, and cook, stirring occasionally, until tender, adding 2 to 3 Tbs. of water if the pan becomes dry, about 15 minutes. Add the wine and simmer until nearly evaporated, about 2 minutes. remove the pan from the heat, fold in remaining 1 tablespoon butter/margarine, and season with salt and pepper. Serve the fish over the leeks.