Preheat your oven to 375°F (190°C).

In a large skillet, combine butter and olive oil over medium heat. Add the sliced onions and cook for 15-20 minutes until caramelized.

Add minced garlic to the skillet and sauté for an additional 2 minutes.

In a mixing bowl, whisk together heavy cream, beef broth, Worcestershire sauce, thyme, salt, and black pepper.

Lightly grease your baking dish. Layer thin slices of potatoes, followed by caramelized onions. Repeat the layers with potatoes on top.

Pour the cream mixture evenly over the layered potatoes and onions.

Cover the baking dish with foil and bake in the oven for 45 minutes.

Remove the foil and sprinkle grated Gruyere and Parmesan cheese on top. Return to the oven and bake uncovered for 15-20 minutes until cheese is golden and bubbly.

Let the bake rest for a few minutes before garnishing with parsley and serving.