Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with butter.

In a large skillet over medium-low heat, melt the butter with olive oil. Add sliced onions and salt; cook for about 30 minutes until golden brown and caramelized.

Stir in garlic powder, thyme, and beef broth; simmer for a few minutes.

Layer half of the potato slices in the greased baking dish. Sprinkle with salt and pepper, then half of the caramelized onion mixture followed by one-third of the Gruyère cheese.

Repeat layers with remaining potatoes, onions, and Gruyère cheese.

In a bowl, whisk together heavy cream, salt, and pepper. Pour over the layered bake.

Cover with aluminum foil and bake for 45 minutes. Remove foil, top with Parmesan cheese, and optional crispy fried onions; bake uncovered for an additional 20-30 minutes until golden brown.