Bright, light, and easy is the name of the game with this simple one-pan fish dinner. It all comes together on a single sheet pan thanks to strategic timing
1 pound small Yukon Gold potatoes, halved
½ cup olive oil, divided
1 ¼ teaspoons kosher salt, divided
¾ teaspoon freshly ground black pepper, divided
4 6-ounce cod fillets
2 bunches Swiss chard, stems removed and leaves chopped
½ cup pitted green olives (such as Castelvetrano), halved
3 tablespoons fresh lemon juice (from 2 lemons)
1 tablespoon chopped fresh flat-leaf parsley leaves
1 tablespoon finely chopped shallot
1 ½ teaspoons Dijon mustard
Preheat oven to 450°F. Toss potatoes with 2 tablespoons oil, ½ teaspoon salt, and ¼ teaspoon pepper on a large rimmed baking sheet; spread in a single layer, cut sides down. Bake until just tender, 15 to 20 minutes.
Meanwhile, rub cod with 1 tablespoon oil and season with ½ teaspoon salt and remaining ½ teaspoon pepper.
Flip potatoes and move to outer edges of baking sheet. Place chard and olives in center of baking sheet, drizzle with 1 tablespoon oil, and top with cod. Bake until potatoes are tender, about 5 minutes. Increase oven temperature to high broil and broil until cod is golden and chard is wilted and slightly charred, 4 to 5 minutes.
Whisk lemon juice, parsley, shallot, mustard, and remaining ¼ cup oil and ¼ teaspoon salt in a small bowl; drizzle over cod mixture.