Preheat your oven to 375°F (190°C).

In a skillet over medium heat, melt 2 tablespoons of butter with 1 tablespoon of olive oil. Add the sliced onions and cook for about 15 to 20 minutes until caramelized.

Stir in 3 minced garlic cloves and cook for an additional 2 minutes.

In a mixing bowl, whisk together 1 cup of heavy cream, 1/2 cup of beef broth, 1 tablespoon of Worcestershire sauce, 1 teaspoon of dried thyme, 1 teaspoon of salt, and 1/2 teaspoon of black pepper.

Grease a 9x13-inch baking dish with butter or oil. Layer slices of Russet potatoes, followed by caramelized onions. Repeat layering, finishing with potatoes.

Pour the cream mixture over the layered potatoes and onions. Tap the dish on the counter to release air pockets.

Cover with aluminum foil and bake for 45 minutes.

Remove the foil, sprinkle with 1 cup of grated Gruyère cheese and 1/2 cup of grated Parmesan cheese, and bake uncovered for an additional 15 to 20 minutes.

Let the dish rest for a few minutes, then garnish with freshly chopped parsley before serving.