1. Prepare the Potatoes

1. Peel the potatoes and slice them into thin, even rounds to ensure uniform cooking.
2. Parboil the potato slices in lightly salted water for about 7-8 minutes, until just tender but not fully cooked.
3. Drain and set aside to cool slightly while preparing the sauce.

2. Sauté the Aromatics

1. In a large skillet or oven-safe pan, heat olive oil and butter over medium heat.
2. Add diced onions and sauté for 5-7 minutes until softened and translucent.
3. Stir in the minced garlic and rosemary, cooking for an additional 1-2 minutes until fragrant.

3. Make the Cream Sauce

1. Pour in the vegetable broth and bring to a gentle simmer.
2. Stir in the heavy cream and Dijon mustard, blending until smooth and slightly thickened.
3. Add salt, pepper, and lemon juice, adjusting seasoning as needed. Simmer for 3-4 minutes, then remove from heat.

4. Assemble the Dish

1. Preheat your oven to 180°C (350°F).
2. In the same pan (or transfer to an oven-safe dish), arrange the parboiled potatoes in an even layer.
3. Place the cod fillets over the potatoes and gently spoon the cream sauce over everything, ensuring full coverage.

5. Bake the Dish

1. Cover the dish with foil and bake for 20 minutes.
2. Remove foil and continue baking for another 10-12 minutes, or until the cod flakes easily with a fork and the sauce is bubbling.

6. Garnish and Serve

1. Sprinkle fresh parsley over the finished dish for color and freshness.
2. Serve hot with crusty bread, steamed vegetables, or a side salad for a well-rounded meal.