Herb-Crusted Baked Cod with Creamy Leek and Potato Gratin and Lemony Asparagus

Preheat the oven to 400°F (204°C). Grease an 8x8-inch baking dish with 1 tbsp of unsalted butter.

Wash 1.5 lbs of Yukon Gold potatoes, peel them, and use a mandoline to slice them into uniform 1/8-inch thick rounds.

Wash 1 large leek, slice the white and light green parts thinly, and rinse thoroughly in cold water to remove any grit. Drain well.

Mince 3 cloves of garlic. Chop 0.25 cup of fresh flat-leaf parsley and 2 tbsp of fresh dill. Zest 1 lemon to yield 1 tsp of zest, then cut the lemon in half to save for juicing.

Melt 2 tbsp of unsalted butter in a medium skillet over medium heat. Add the sliced leeks and minced garlic. Sauté for 5 minutes until soft and translucent, then season with 1/2 tsp of salt and 1/4 tsp of black pepper.

In a mixing bowl, whisk together 1 cup of heavy cream, the leaves stripped from 3 sprigs of fresh thyme, 1/2 tsp of salt, and 1/4 tsp of black pepper. Stir the sautéed leeks and garlic into the cream mixture.

Layer half of the sliced potatoes in the greased baking dish. Pour half of the cream and leek mixture over the potatoes, and sprinkle with 0.5 cup of shredded Gruyere cheese. Layer the remaining potatoes on top, pour the remaining cream mixture over them, and finish with the remaining 0.5 cup of Gruyere cheese.

Cover the gratin dish tightly with aluminum foil and bake in the preheated oven for 25 minutes.

While the gratin is baking, prepare the herb crust for the fish. In a small bowl, combine 0.5 cup of panko breadcrumbs, the chopped parsley, the chopped dill, 1 tsp of lemon zest, and 1 tbsp of melted unsalted butter. Season with a pinch of salt and black pepper.

Trim the woody ends off 1 lb of asparagus. Toss the asparagus with 1 tbsp of olive oil, a pinch of salt, and a pinch of black pepper. Spread the asparagus out on one side of a large rimmed baking sheet.

Pat dry 3 raw cod fillets with paper towels. Season both sides of the fillets with salt and black pepper. Place the fillets on the empty side of the baking sheet.

Brush the top of each cod fillet with 1 tsp of Dijon mustard, then press the herb-panko mixture firmly onto the mustard-coated tops to create an even crust.

After the gratin has baked for 25 minutes, carefully remove the aluminum foil. Place the baking sheet with the cod and asparagus into the oven alongside the uncovered gratin.

Bake everything for 15 minutes, until the cod's herb crust is golden brown and the fish flakes easily with a fork (internal temperature of 145°F/63°C), the asparagus is tender-roasted, and the gratin is bubbly and deeply browned on top.

Remove all dishes from the oven. Squeeze 1 tbsp of fresh lemon juice over the roasted asparagus. Let the gratin rest for 5 minutes to set before slicing and serving alongside the herb-crusted cod and lemony asparagus.