01 - Preheat oven to 375°F. Lightly grease a 9x13-inch casserole dish with olive oil or nonstick spray.
02 - In a large skillet over medium heat, sauté the chopped onion and diced red bell pepper in a drizzle of olive oil for 4 to 5 minutes until softened. Add minced garlic and cook until fragrant, about 1 minute longer.
03 - Add chopped spinach to the skillet and cook, stirring, until wilted. Remove the pan from the heat.
04 - In a large mixing bowl, combine the cooked chicken, uncooked long-grain white rice, sautéed vegetables, crumbled feta, shredded mozzarella, dried oregano, thyme, salt, and black pepper. Mix until ingredients are evenly distributed.
05 - In a separate bowl, whisk together the whole milk, sour cream, and chicken broth until smooth.
06 - Pour the dairy and broth mixture over the chicken and rice mixture. Stir thoroughly to ensure all ingredients are well combined.
07 - Evenly spread the mixture into the prepared 9x13-inch casserole dish, smoothing the top with a spatula.
08 - If desired, mix breadcrumbs with 2 tablespoons olive oil and sprinkle evenly over the top of the casserole.
09 - Cover the casserole with aluminum foil and bake for 30 minutes.
10 - Remove foil and continue to bake for an additional 15 minutes, or until the top is golden and the casserole is bubbling.
11 - Allow the casserole to rest for 5 to 10 minutes before serving.