1. Preheat your oven to 450°F. This high heat is crucial for achieving those beautifully crispy edges.
2. In a large oven-safe skillet, melt the butter over medium heat. Add the sliced onions and cook until they become golden and caramelized, about 15 minutes. Stir occasionally to prevent burning. My abuela always said patience is key when it comes to onions!
3. Add the minced garlic to the onions and sauté for another minute until fragrant.
4. Arrange the potato slices in the skillet, ensuring they overlap slightly. Season generously with salt and pepper.
5. Pour the beef broth over the potatoes and sprinkle with fresh thyme. Transfer the skillet to the preheated oven.
6. Roast for 35-40 minutes, flipping the potatoes halfway through, until they are tender and starting to brown.
7. Remove the skillet from the oven and sprinkle the Gruyère cheese over the top. Return to the oven for an additional 5 minutes, or until the cheese is melted and bubbly.
8. Garnish with fresh parsley before serving. This is your moment to bask in the delicious aroma that fills your kitchen!

These steps are crafted with care, much like how I learned to cook alongside my grandmother, focusing on intuition and taste rather than strict rules. Feel free to tweak the recipe to suit your personal taste and what you have on hand.