In a large oven-safe skillet or Dutch oven, melt 2 tablespoons of unsalted butter and 1 tablespoon of olive oil over medium-low heat. Add 2 large yellow onions sliced thinly, along with a sprinkle of sugar and salt. Cook the mixture for 30–40 minutes, stirring occasionally, until the onions are golden and caramelized. In the last minute, stir in 2 minced garlic cloves until fragrant.

While the onions are caramelizing, preheat another skillet over medium heat and season 2 cups of cooked shredded chicken with salt and pepper. Add the chicken to the skillet, browning it for about 5–7 minutes until golden.

Carefully deglaze the onion mixture by pouring in 1 cup of beef broth, scraping up any delicious brown bits from the bottom of the skillet. Stir in 1 cup of orzo pasta, 1 cup of heavy cream, and 1 teaspoon of dried thyme. Add the browned chicken and bring the mixture to a gentle simmer for 5–7 minutes.

Preheat your oven to 375°F (190°C). Mix half of the 1 cup of shredded mozzarella cheese into the skillet, then transfer the entire mixture to a baking dish. Sprinkle the remaining mozzarella on top, and bake uncovered for 20–25 minutes, or until the casserole is bubbling and the cheese is golden and melted.

Remove the onion chicken orzo casserole from the oven and let it rest for 5–10 minutes before serving. Garnish with fresh thyme or parsley if desired.