Creamy baked fish on potato gratin

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. (Note 1) 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! Eat the fish and gratin together with each mouthful for maximum eating pleasure!!

Ingredient :

4 x 160g/5oz white fish fillets , ~2cm / 0.8" thick, skinless (barramundi, snapper, tilapia, cod, salmon – Note 1)
1/2 tsp cooking/kosher salt
1/4 tsp black pepper
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:
3/4 cup panko breadcrumbs
2 tbsp extra virgin olive oil
1/4 cup parmesan