Call it a classy casserole—or maybe fancy fish pie. Firm white cod gets the luxury treatment when you sink it into sweet leeks and mushrooms, and top it with a thick crust of buttery crumbs.
Allergen Note: Contains milk, wheat, fish.
Preheat the oven to 425°F.
To prep the leeks, trim the roots and tops, reserving the white and pale green parts. Cut the leeks in half lengthwise, then crosswise into ½-inch pieces (you should have about 2 cups). Rinse in cool water and pat or shake dry.
In a large ovenproof sauté pan over medium-high heat, melt 3 tablespoons butter. Add the leeks and mushrooms, along with a pinch of salt, and sauté until soft, 12 to 15 minutes. Add ½ cup of cream and ¼ cup white wine or water and simmer until thickened slightly, 2 minutes.
Meanwhile, pick the parsley, toss the stems, and chop the leaves. In a bowl, stir together the panko, chopped parsley, 1 tablespoon olive oil, and a pinch of salt.
Remove the cod from the packaging, pat dry with paper towels, and season with salt. When the mushrooms and leeks are ready, add the cod to the pan, placing it on top of the vegetables, overlapping it slightly, if necessary. Sprinkle the panko mixture over the top. Transfer the pan to the oven and bake until the fish is firm and flakes easily when pierced with a fork, and the panko is golden, 8 to 10 minutes, depending on the thickness of the fish.
Transfer the cod to plates, scooping it up with the leeks and mushrooms. Finish with a squeeze of lemon and serve warm.
The cod may arrive frozen for freshness. Store it in the refrigerator to defrost before cooking. If you need to defrost it same day, place the sealed package in a bowl of cold water, and leave the tap running in a thin stream for about 30 minutes.