Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch casserole dish.
Cook the orzo pasta according to package instructions until al dente. Drain and set aside.
Steam the broccoli florets for 4-5 minutes until bright green and slightly tender. Drain and set aside.
Heat olive oil in a large skillet over medium-high heat. Add the chicken pieces and cook for 5-7 minutes until cooked through and lightly browned. Remove chicken from skillet and set aside.
In the same skillet, add the chopped onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for an additional 1 minute until fragrant.
In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes until lightly golden and bubbling.
Gradually whisk in the milk and chicken broth, stirring constantly to avoid lumps. Continue cooking and whisking until the sauce thickens, about 5-7 minutes.
Remove the sauce from heat and stir in salt, black pepper, paprika, dried thyme, and 1 cup of shredded cheddar cheese until melted and smooth.
In a large mixing bowl, combine the cooked orzo, steamed broccoli, cooked chicken, sautéed onions and garlic, and the cheese sauce. Mix well to evenly distribute all ingredients.
Pour the mixture into the prepared casserole dish and spread evenly.
Sprinkle the remaining 1/2 cup shredded cheddar cheese, grated Parmesan cheese, and breadcrumbs evenly over the top.
Bake in the preheated oven for 20-25 minutes until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it rest for 5 minutes before serving.