Heat a large oven-safe skillet over medium. Add the butter and olive oil and wait until the butter melts.

Add the sliced onions, sugar, and salt. Cook slowly, stirring occasionally, for 20 to 25 minutes until they are deeply caramelized and jammy. Lower the heat if they begin to brown too quickly.

Stir in the minced garlic and cook for 1 minute until fragrant.

Add the orzo and toast it in the pan for about 2 minutes, stirring so it picks up the onion flavor.

Stir in the shredded chicken, thyme, and black pepper. Add the Worcestershire sauce if using.

Pour in the chicken broth and heavy cream. Bring the mixture to a gentle simmer, then cover and cook for 8 to 10 minutes, until the orzo is tender but not mushy.

Remove the lid. Stir in 1 cup of the mozzarella and all of the Parmesan until melted and evenly combined. Taste and adjust seasoning.

Preheat your oven to 375°F (190°C). Sprinkle the remaining mozzarella evenly over the top.

Transfer the skillet to the oven and bake for 10 to 15 minutes, until the cheese is bubbly and golden.

Let the casserole rest for 5 to 10 minutes before serving so it firms up for neat portions.