In a large oven-safe skillet, melt 2 tablespoons of unsalted butter and 1 tablespoon of olive oil over medium-low heat. Add 3 sliced yellow onions and cook, stirring often for 25–30 minutes, until they are deeply golden brown and beautifully caramelized.

Once the onions are caramelized, stir in 2 minced garlic cloves, 1 tablespoon of fresh thyme, and a pinch of salt and black pepper. Cook for another minute until the garlic is fragrant, then deglaze the pan with 1 tablespoon of balsamic vinegar.

Add 1 cup of dry orzo pasta to the skillet, stirring gently to incorporate it well with the onion mixture. Pour in 3 cups of beef broth and bring the mixture to a simmer over medium heat. Allow it to cook uncovered for 8–10 minutes, stirring occasionally.

Reduce the heat to low, and stir in half of the shredded Gruyère and mozzarella cheeses until fully melted and creamy. Sprinkle the remaining cheeses on top.

Preheat your oven to 400°F (200°C). Once heated, transfer the skillet to the oven and bake for 10–15 minutes until bubbly and golden-brown.

After baking, allow the dish to cool for a few minutes. Garnish with freshly chopped parsley or chives over the top, and serve warm.