Melt the butter and olive oil in a large oven-safe skillet or Dutch oven over medium heat. Add the sliced onions, sugar, and salt. Cook for 20–25 minutes, stirring occasionally, until the onions are deeply golden and caramelized. In the last 1–2 minutes, stir in the minced garlic for added aroma.

Toast the orzo by stirring it into the pan. Cook for 2 minutes, allowing it to lightly toast and absorb flavors. Then, add the shredded chicken, thyme, black pepper, and Worcestershire sauce, mixing everything well.

Combine the chicken broth and heavy cream into the mixture. Bring it to a gentle simmer. Lower the heat, cover, and let it cook for about 8–10 minutes, stirring occasionally, until the orzo is perfectly tender.

Incorporate 1 cup of mozzarella and the grated Parmesan cheese into the mix. Stir until everything is melted and creamy, embodying that delicious comfort food essence.

Preheat the oven to 375°F (190°C). Sprinkle the remaining ½ cup of mozzarella over the casserole's top. Bake uncovered for 10–15 minutes until the surface is bubbly and golden brown.

Rest the casserole for 5–10 minutes after baking before serving. If desired, garnish with fresh thyme or parsley for a pop of color and flavor.