Preheat Oven: Preheat your oven to 400°F (200°C). Grease a 9x13-inch baking dish.
Prepare Potatoes and Onions: In a large bowl, combine the sliced potatoes and onions. Drizzle with olive oil and sprinkle with the French onion soup mix. Toss until the potatoes and onions are evenly coated.
Layer in Baking Dish: Arrange the potato and onion mixture in the prepared baking dish in an even layer.
Add Cheese and Cream: Pour the heavy cream evenly over the potatoes and sprinkle with shredded Gruyère or Swiss cheese.
Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes. Then, remove the foil and bake for an additional 15-20 minutes, or until the potatoes are tender and the top is golden and bubbly.
Garnish and Serve: Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.