Heat the olive oil in a large skillet over medium-high heat.
Season the chicken breasts evenly with salt, black pepper, and garlic powder.
Add the chicken breasts to the skillet and cook for 5-6 minutes on each side, or until golden brown and cooked through (internal temperature should reach 165°F). Remove the chicken from the skillet and set aside.
Reduce the heat to medium. Add the chopped onion to the same skillet and sauté for 3-4 minutes until softened.
Add the minced garlic and diced zucchini to the skillet. Cook for an additional 4-5 minutes until the zucchini is tender.
Stir in the rinsed rice, coating it well with the vegetables and oil.
Pour in the chicken broth, then add dried thyme and crushed red pepper flakes if using. Stir to combine.
Bring the mixture to a boil, then reduce heat to low and cover the skillet with a lid.
Simmer for 15 minutes, or until the rice is tender and the liquid is absorbed.
Remove the lid and stir in the cheddar cheese, mozzarella cheese, and Parmesan cheese until melted and creamy.
Return the cooked chicken breasts to the skillet on top of the cheesy rice and zucchini mixture.
Cover the skillet again and cook for 2-3 minutes to warm the chicken through.
Garnish with chopped fresh parsley before serving.