Preheat your oven to 375°F. Grease a 9×13-inch baking dish with cooking spray or butter.
In a large bowl, combine the sliced potatoes and onions.
In another bowl, whisk together the heavy cream, garlic powder, salt, and black pepper. Pour the cream mixture over the potatoes and onions, stirring to combine.
Layer half of the potato mixture in the bottom of the prepared baking dish. Sprinkle half of the shredded cheddar cheese over the potatoes.
Add the remaining potato mixture on top and finish with the rest of the cheese.
In a separate bowl, drizzle the fish fillets with olive oil and lemon juice, then season with salt and pepper. Place the fish fillets on top of the potato gratin.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the fish is cooked through and the potatoes are tender and golden brown.
Let the dish cool for a few minutes before serving. Garnish with fresh parsley.
Notes
For a lighter version, substitute half-and-half for the heavy cream.
You can also add vegetables like spinach or broccoli to the potato layer for added nutrition.
If you prefer a crispy topping, sprinkle some breadcrumbs mixed with a little melted butter on top before the final baking.