Whole Baked Trout with Fennel and Orange

Name: Whole Baked Trout with Fennel and Orange
Ingredients: 4 tablespoons unsalted butter, plus 3 tablespoons melted; 1 small onion, finely chopped; 1/2 fennel bulb—trimmed, cored and thinly sliced crosswise; 3 scallions, coarsely chopped; 5 garlic cloves, crushed; 2 teaspoons chopped thyme; 1 tablespoon dried lovage or 1 teaspoon celery seeds; One 3-inch strip of orange zest plus 3 tablespoons fresh orange juice; 1 teaspoon light brown sugar; 1 cup dry white wine; One 4-pound steelhead trout or arctic char—cleaned, rinsed and patted dry; 2 tablespoons extra-virgin olive oil; Flaky sea salt; Pepper; 2 tablespoons honey-Dijon mustard; 1 orange, thinly sliced