Preheat oven to 200C
Place the potatoes & Onions in a baking dish lined with baking paper
Spread potatoes & Onions out evenly and drizzle with half the oil and a little salt and pepper if desired
Bake the potatoes for 15-20 minutes or until just tender.
Meanwhile, combine breadcrumbs, parsley, parmesan, lime rind and juice, and salt and pepper in a bowl.
Stir to combine. Drizzle mixture with oil and stir until breadcrumbs are coated in oil
Press breadcrumb mixture onto flesh-side of fish fillets to form an even topping
Remove potatoes from oven. Place the fish fillets skin-side down, on top of potatoes.
Bake for a further 15 minutes or until crumbs are light golden and fish is just cooked through.