Preheat your oven to 400°F (200°C). In a large pot, bring salted water to a boil. Add the halved baby potatoes and cook for about 10 minutes, or until just tender. Drain and set aside.
In a large oven-safe skillet, heat the olive oil over medium heat. Season the cod fillets with salt and black pepper, then add them to the skillet. 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, add the minced garlic and cook for about 30 seconds until fragrant. Pour in the heavy cream and stir in the chopped rosemary and lemon juice. Bring the mixture to a simmer.
Add the cooked potatoes to the skillet, stirring gently to coat them in the cream sauce. Place the cod fillets back in the skillet on top of the potatoes.
Sprinkle the grated Parmesan cheese over the cod and potatoes. 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. Enjoy your creamy rosemary cod with potatoes!