Start by placing a rack in the center of your oven and preheat it to 350 degrees F. This ensures even cooking for your casserole.
Next, coat a 9×13 or a similar casserole dish with nonstick spray to prevent sticking. It’s always a good idea to ensure your dish is ready for all that cheesy goodness!
In a Dutch oven or a large, sturdy-bottomed pot, heat olive oil and butter over medium heat. As the butter melts and begins to foam, it’s the perfect time to add your onion, salt, and pepper. Cook this mixture until the onion is softened and translucent, about 8 to 10 minutes. Stir occasionally and adjust the heat as needed to avoid browning the onions, which can affect the flavor.
Once your onions are looking lovely, stir in the garlic and let it cook for about 30 seconds. The moment you add the garlic, you’ll notice that delightful aroma wafting through the kitchen!
Now, sprinkle the flour over the onion and garlic mixture. Cook and stir constantly for at least 1 full minute until the flour starts to turn brown and no white bits remain. This adds a rich flavor base to your sauce.
Slowly pour in the chicken broth and milk, stirring continuously to prevent any lumps from forming. Take your time with this step; it’s key to a smooth and creamy sauce. Increase the heat to bring the mixture to a gentle simmer, then reduce it and let it gently bubble until the sauce thickens slightly, about 5 minutes. Stir often and adjust the heat as necessary.
After your sauce has thickened, remove the pan from the heat. Stir in the mixed vegetables, Greek yogurt, half of the cheddar cheese, mozzarella, Dijon mustard, and thyme until everything is smooth and well combined. This is the moment when your casserole starts taking shape!
Next, gently fold in the shredded chicken and cooked rice, mixing thoroughly to ensure every bite is packed with flavor.
Transfer the creamy mixture to your prepared casserole dish, spreading it evenly. Top it off with the remaining cheddar cheese, creating a delicious cheese blanket.
Finally, bake the casserole in the preheated oven until the cheese is melted and the casserole is hot, about 15 minutes. For a beautiful golden top, turn the oven to broil and continue cooking for about 3 more minutes, watching carefully to prevent burning. Once done, remove it from the oven and sprinkle with fresh parsley or thyme for a pop of color.
Allow the casserole to rest for 5 minutes before serving. This will help the layers settle and make it easier to serve!
