Step 1

Preheat the oven to 425° and line a large, rimmed baking sheet with parchment paper.

Step 2

In a bowl, toss the potatoes and onions with 1 1/2 tablespoons of the olive oil and 4 thyme sprigs and season with salt and pepper. Spread on a large rimmed baking sheet and roast for 20 minutes, until the potatoes are blistered in spots and barely tender.

Step 3

Rub both fish all over with the remaining 1 1/2 tablespoons of oil; season generously with salt. Stuff the cavities with the lemon slices and the remaining 8 thyme sprigs. Stir the potatoes, pushing them toward the sides, and set the fish on the baking sheet. Roast for 30 minutes, until the fish are just cooked through and the potatoes and onions are tender.

Step 4

Transfer the potatoes and onions to a platter. Using two forks, pull the skin off the top of the fish and discard. Remove the fillets from the bones and transfer to the platter. Squeeze the lemon juice over the fish, potatoes and onions, sprinkle with the parsley and serve.