The key to making this cooking method work is to use a heavy skillet—cast-iron is perfect if you have one—and heating it until it’s searingly hot. The initial roasting time for the potatoes, spreading them out in a single layer, gives them a head start to becoming crisp on the outside and creamy in the center. If you use 1-Bite potatoes you don’t have to cut them in half. Make sure you use good oven mitts to remove the hot skillet from the oven as you add and turn the food.
To serve alongside the fish and potatoes, trim 1 lb. fresh asparagus or a large bunch of small spring carrots with tops (trim the tops and slice them in half lengthwise.) Spread them out on a parchment-lined rimmed baking sheet. Drizzle with 2 tablespoons olive oil and season with salt and freshly ground pepper. Slide the pan onto the rack under the cast-iron skillet and roast for 20-25 minutes or until tender and browned. Sprinkle with some halved small cherry tomatoes or a handful of fresh chopped tarragon leaves.