Preheat the oven to 375°F (190°C).
Place the diced potatoes in a large pot and cover with water. Bring to a boil over high heat and cook for 8-10 minutes until just tender. Drain and set aside.
In a large skillet, heat the olive oil over medium-high heat. Add the chicken pieces and cook for 5-6 minutes until browned and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the diced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for 1 minute until fragrant.
In a large mixing bowl, combine the cooked potatoes, cooked chicken, sautéed onions and garlic, frozen peas, frozen corn, shredded cheddar cheese, shredded mozzarella cheese, condensed cream of chicken soup, sour cream, milk, dried thyme, paprika, salt, and black pepper. Stir until all ingredients are evenly mixed.
Transfer the mixture to a greased 9×13-inch baking dish and spread evenly.
Bake uncovered in the preheated oven for 35-40 minutes until the casserole is bubbly and the top is golden brown.
Remove from the oven and let it rest for 5 minutes. Sprinkle with chopped fresh parsley before serving.