Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook until softened, about 3-4 minutes.
Stir in the minced garlic and cook for another 1 minute until fragrant.
Add the diced chicken to the skillet and season with paprika, salt, black pepper, and cayenne pepper if using. Cook for about 5-7 minutes until the chicken is cooked through and no longer pink.
Pour in the chicken broth and milk, stirring to combine. Bring the mixture to a simmer.
Add the cooked rice and frozen mixed vegetables to the skillet, stirring until everything is well combined. Let it simmer for about 5 minutes, allowing the flavors to merge.
Remove the skillet from heat and stir in the shredded cheddar cheese until melted and creamy.
Adjust the seasoning to taste if necessary, then cover and let sit for a few minutes before serving.