Heat the olive oil in a large skillet over medium-high heat.
Add the chicken pieces to the skillet and cook for 5-6 minutes, stirring occasionally, until lightly browned but not fully cooked. Remove the chicken from the 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 30 seconds until fragrant.
Stir in the rinsed rice and cook for 1-2 minutes to lightly toast the rice.
Pour in the chicken broth and milk, then stir to combine.
Add the thawed mixed vegetables, dried thyme, paprika, salt, and black pepper. Stir well.
Return the browned chicken pieces to the skillet, distributing them evenly.
Bring the mixture to a gentle boil, then reduce the heat to low.
Cover the skillet with a tight-fitting lid and simmer for 20-25 minutes, or until the rice is tender and the liquid is absorbed.
Remove the lid and sprinkle the shredded cheddar cheese evenly over the top.
Cover again and cook for an additional 3-5 minutes until the cheese is melted.
Remove from heat and let the casserole rest for 5 minutes before serving.