In a large pot, bring salted water to a boil. Add the diced potatoes and cook for about 10-12 minutes, or until fork-tender. Drain and set aside.
While the potatoes are cooking, season the cod fillets with salt and pepper. In a large oven-safe skillet, heat the olive oil over medium-high heat. Add the cod fillets and sear for about 3-4 minutes on each side until golden brown. Remove the cod from the skillet and set aside.
In the same skillet, reduce the heat to medium and add the butter. Once melted, add the minced garlic and sauté for about 1 minute until fragrant.
Pour in the heavy cream and stir in the rosemary and lemon juice. Bring the mixture to a gentle simmer, allowing it to thicken slightly for about 3-5 minutes.
Return the seared cod to the skillet and nestle it among the potatoes. Spoon the rosemary cream sauce over the top.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the cod is cooked through and flakes easily with a fork.
Remove from the oven and let it cool for a few minutes before serving.
Notes
For a lighter version, substitute half of the heavy cream with low-fat milk or a non-dairy alternative.
Add steamed green beans or asparagus to the skillet before baking for extra vegetables and color.