Leeks braised in orange juice perfume and brighten a fillet of zest-infused striped bass.
Preheat oven to 375 degrees. Using 1 tablespoon oil, brush bottom of an 8-inch square baking dish and an 8-inch square of parchment paper. Place leeks in dish, cut sides up, alternating root ends with tops for a snug fit. Season with 1/2 teaspoon salt and a pinch of pepper. Arrange half the thyme and half the orange slices over leeks, and pour orange juice over top. Cover with parchment paper, oiled side down. Braise until leeks are tender, about 30 minutes.
Season fish with 1/4 teaspoon salt and a pinch of pepper. Remove parchment. Arrange fish over orange slices. Top with olives and remaining thyme and orange. Cover with the parchment; bake until fish flakes easily, about 15 minutes.
Divide fish, olives, and orange slices among plates. Season leeks with remaining 1/4 teaspoon salt, and add to plates. Garnish with thyme sprigs.