Heat 1 tablespoon of olive oil in a large skillet over medium-high heat.
Season the chicken breasts evenly with kosher salt, black pepper, and garlic powder on both sides.
Add the chicken breasts to the skillet and cook for 5-6 minutes on each side until golden brown and cooked through (internal temperature 165°F). Remove chicken from the skillet and set aside.
Reduce heat to medium and add the remaining 1 tablespoon of olive oil to the skillet.
Add the chopped onion and sauté for 3-4 minutes until translucent.
Add the minced garlic and diced zucchini, cooking for another 4-5 minutes until zucchini is tender.
Stir in the rinsed rice and cook for 1-2 minutes, stirring frequently to coat the rice in the oil and vegetables.
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 tight-fitting lid.
Simmer for 15 minutes, or until the rice is tender and liquid is absorbed.
Remove the skillet from heat and stir in the shredded cheddar, mozzarella, and Parmesan cheeses until melted and creamy.
Return the cooked chicken breasts to the skillet, nestling them into the cheesy rice and zucchini mixture.
Cover and let rest for 5 minutes to allow flavors to meld and chicken to warm through.
Sprinkle chopped fresh parsley over the top before serving.