Step 1: Preheat your oven to 400°F (200°C). In a large bowl, toss the potato pieces with 1 tablespoon of olive oil, salt, and pepper. Spread them in a single layer on a baking sheet.
Step 2: Roast the potatoes for 20-25 minutes, or until tender and lightly browned, tossing once halfway through.
Step 3: While potatoes roast, pat cod fillets dry with paper towels and season both sides with salt and pepper.
Step 4: In a large oven-safe skillet, heat the remaining 1 tablespoon of olive oil and butter over medium-high heat. Sear the cod for 2-3 minutes per side until golden. Remove cod from the skillet and set aside.
Step 5: Reduce heat to medium. Add minced garlic to the skillet and sauté for 1 minute until fragrant. Stir in the chopped fresh rosemary and cook for another 30 seconds.
Step 6: Pour in the chicken broth, scraping up any browned bits from the bottom of the skillet. Bring to a simmer and reduce slightly, about 1-2 minutes.
Step 7: Whisk in the heavy cream and Dijon mustard. Bring the sauce to a gentle simmer and cook for 5-7 minutes, stirring occasionally, until it thickens to your desired consistency. Season with salt and pepper to taste.
Step 8: Gently return the roasted potatoes to the skillet, tossing to coat. Nestled the seared cod fillets into the sauce and potatoes. Spoon some sauce over the cod.
Step 9: Transfer the skillet to the preheated oven and bake for 10-15 minutes, or until the cod is cooked through and flakes easily with a fork (internal temperature 145°F/63°C).
Step 10: Garnish with fresh chopped parsley or extra rosemary sprigs, if desired, and serve immediately.