For the Caramelized Onions:

• 2 large yellow onions, thinly sliced
• 2 tablespoons butter
• 1 tablespoon olive oil
• 1 teaspoon salt
• 1/2 teaspoon black pepper
• 1 teaspoon sugar (optional)
• 1/2 teaspoon garlic powder
• 1/2 teaspoon dried thyme (or 1 teaspoon fresh thyme)
• 1/2 cup beef broth (or vegetable broth for vegetarian version)

For the Potato Bake:

• 4 large Yukon Gold potatoes, sliced 1/4-inch thick
• 1 tablespoon butter (for greasing)
• 1 cup shredded Gruyère cheese (or Swiss, cheddar, or mozzarella)
• 1/2 cup Parmesan cheese, grated
• 1 cup heavy cream or whole milk
• 1/2 teaspoon salt
• 1/4 teaspoon black pepper
• 1 teaspoon fresh thyme leaves

For Garnishing:

• Fresh thyme sprigs
• Extra Parmesan cheese
• Crispy fried onions (optional)