Preparing a Chicken and Vegetable Casserole is straightforward when you follow these steps:

1. Preheat your oven to 375°F (190°C).
2. Cook the chicken:
   • In a large pan, heat olive oil over medium-high heat.
   • Add the cubed chicken, season with salt and pepper, and cook until browned and cooked through, about 6-8 minutes. Remove from the pan and set aside.
3. Sauté vegetables:
   • In the same pan, add onion and garlic. Sauté until the onion is translucent.
   • Add broccoli, carrots, and bell pepper. Cook for another 5 minutes until the vegetables begin to soften.
   • Stir in the frozen peas and cook for an additional 2 minutes.
4. Prepare the sauce:
   • In a medium saucepan, whisk together the chicken broth, milk, and heavy cream over medium heat.
   • Gradually whisk in the flour until the mixture thickens, about 4-5 minutes.
   • Stir in the dried thyme, oregano, salt, and pepper.
5. Combine ingredients:
   • In a large mixing bowl, combine the cooked chicken, sautéed vegetables, cooked rice, and sauce. Stir until well mixed.
   • Transfer the mixture to a greased 9×13-inch baking dish.
6. Top the casserole:
   • Sprinkle the shredded cheddar cheese evenly over the top.
   • In a small bowl, mix the breadcrumbs with melted butter and sprinkle over the cheese layer.
7. Bake:
   • Place the casserole in the preheated oven and bake for 25-30 minutes, or until the cheese is bubbly and the top is golden brown.
8. Serve:
   • Remove from the oven and let cool slightly before serving.