Ikan krim di atas kentang gratin

Bahan-bahan (4 porsi)

• 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
• 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 g arlic 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
• 3/4 cup panko breadcrumbs
• 2 tbsp extra virgin olive oil
• 1/4 cup parmesan (, finely shredded)