Preheat the oven to 375°F (190°C).
In a medium saucepan, bring 2 cups of water to a boil. Stir in the Knorr Chicken Flavor Rice Mix, reduce heat to low, cover, and simmer for 15 minutes or until rice is tender and water is absorbed.
While the rice cooks, heat olive oil in a large skillet over medium heat.
Add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Add the bite-sized chicken pieces to the skillet. Season with black pepper, paprika, dried thyme, and salt to taste. Cook for 6-8 minutes, stirring occasionally, until chicken is cooked through and no longer pink inside.
Add frozen mixed vegetables to the skillet and cook for 3-4 minutes until heated through.
In a large mixing bowl, combine the cooked rice, chicken and vegetable mixture, sour cream, milk, and half of the shredded cheddar and mozzarella cheeses. Stir until well combined.
Transfer the mixture into a greased 9×13-inch baking dish and spread evenly.
Sprinkle the remaining cheddar and mozzarella cheese evenly over the top.
Bake uncovered in the preheated oven for 20 minutes or until the casserole is bubbly and the cheese is melted and golden brown.
Remove from the oven and let it rest for 5 minutes before serving.