Cut off roots and trim dark-green top from leek. Discard any tough outer leaves. Thinly slice leek. Rinse leek thoroughly in bowl of cold water; swish to remove any sand. With hands, transfer leek to colander, leaving sand in bottom of bowl. Drain well.
Preheat oven to 425°F. Spray 13" by 9" glass baking dish with nonstick cooking spray. To baking dish, add leek, potatoes, fennel, 1 table¬spoon oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper; toss to coat, then spread evenly. Roast vegetables 35 minutes or until tender, stirring once halfway through roasting.
Remove baking dish from oven. Place halibut on vegetables; drizzle with liqueur and remain¬ing 1 teaspoon oil. Sprinkle with fennel seeds, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Place lemon slices on halibut; return dish to oven and roast 10 to 12 minutes or just until halibut turns opaque in center. To serve, sprinkle with fennel fronds, if using.
Recipe courtesy of Good Housekeeping Light & Healthy Cooking: 250 Delicious, Satisfying, Guilt-Free Recipes by The Editors of Good Housekeeping/Hearst, 2012.