Begin by cutting the Yukon gold potatoes into 1-inch pieces, ensuring even cooking. Chop the shallot and mince the garlic. Lastly, cut the cod fillet into four equal pieces.

Preheat your oven to 450°F, setting the stage for roasting. Grease an 8×8-inch baking dish with 1 tablespoon of extra-virgin olive oil.

Toss the prepared potatoes with 1 tablespoon of olive oil, 1 teaspoon of kosher salt, and freshly cracked black pepper. Roast for about 20-25 minutes, or until fork-tender and lightly golden.

Heat 2 tablespoons of salted butter and 1 tablespoon of olive oil in a saucepan over medium heat. Cook the chopped shallot until soft, about 5 minutes. Add minced garlic and stir until fragrant, about 1 minute.

Stir in 1 cup of heavy cream and ½ cup of chicken or fish stock into the shallot mixture. Add rosemary, lemon zest, lemon juice, Dijon mustard, cayenne pepper, and capers. Simmer and thicken the sauce for about 5 minutes.

Sprinkle 1 teaspoon of salt over the cod pieces, nestling them among the potatoes. Pour the creamy rosemary sauce over the top.

Bake uncovered for about 10-15 minutes, or until the cod flakes easily with a fork and is opaque white.

Remove from the oven and let it rest for a minute. Optional: sprinkle with minced flat-leaf parsley before serving.