Creamy baked fish on potato gratin

Ingredients

Fish

Potato gratin

3 tbsp / 50g unsalted butter
2 large leeks, white and pale green part only, washed, cut in half lengthways, sliced 5mm / 0.2" thick OR 2 onions (Note 2)
3 garlic cloves, minced
2 medium starchy potatoes (250g / 8oz each), peeled (Note 3)
3/4 tsp cooking/kosher salt
1/2 tsp black pepper
1/2 cup dry white wine, any type, sub more veg stock (Note 4)
1/2 cup vegetable stock/broth, low sodium
1 cup thickened/heavy cream

Crunchy topping

Oven: Preheat the oven to 180°C/350°F (160°C fan) with a shelf in the middle.
Potatoes: Cut the potatoes into quarters, then sliced 4mm / 1/6" thick. Place cut potato in a large bowl of water (prevents from going brown, removes excess starch so it cooks quicker). Drain before use.
Crunchy topping: Mix panko and olive oil in a bowl. Then stir in parmesan.
Season fish: Sprinkle salt and pepper on both sides of the fish fillets. Set aside.
Sauté: Melt butter in a large oven-proof skillet (30cm/12") over high heat until foamy. Add leek and garlic, then cook for 3 minutes.
Sauté potatoes: Add potato, salt and pepper. Cook for 6 minutes, stirring regularly. The potatoes should be half cooked at this stage.
Reduce wine and stock: Pour in white wine. Simmer rapidly until it is mostly evaporated, about 1 minute. Pour in stock, then simmer for 1 1/2 minutes or until reduced by half.
Top with fish: Turn the stove off. Place fish on top of the potatoes, presentation side up. Pour cream all over, aiming for full coverage. Sprinkle with panko breadcrumbs.
Bake for 30 minutes: I know this sounds like a long time but the fish stays succulent!
Colour topping: Switch to the oven grill (broiler) on high and move the skillet up to the top shelf. Grill/broil 4 minutes or until golden.
Rest: Remove and rest for 5 minutes, then serve!