Preheat your oven to 400°F (200°C).
Dice the potatoes into 1-inch cubes.
In a 9x13 or oval casserole dish, combine the condensed French onion soup, dry onion soup mix, and melted butter.
Add the diced potatoes to the casserole dish and stir to coat them thoroughly in the onion mixture.
Cover the casserole dish with aluminum foil and bake for about 30 minutes.
After 30 minutes, remove the foil and bake for an additional 40 minutes.
Once the potatoes are cooked, sprinkle a generous layer of shredded Gruyere cheese over the top and bake for an additional 10 minutes.
Let it cool for about 10 minutes, garnish with chopped parsley, and serve.