Preheat your oven to 375°F.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 5 minutes until translucent. Stir in the minced garlic and cook for an additional minute until fragrant.
In a large mixing bowl, combine the cooked chicken, rice, chicken broth, milk, frozen vegetables, thyme, paprika, salt, and pepper. Mix well to combine all ingredients.
Transfer the mixture to a greased 9x13-inch baking dish. Cover the dish tightly with aluminum foil.
Bake in the preheated oven for 45 minutes. After 45 minutes, remove the foil, sprinkle the shredded cheddar cheese on top, and return to the oven uncovered for an additional 15 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for about 5 minutes before serving.