Preheat your oven to 350°F (175°C).

In a large skillet, heat olive oil over medium heat and sauté the sliced onions until they are soft and translucent, about 5-6 minutes.

Add the chicken breasts to the skillet, and season with garlic powder, salt, and pepper. Cook until the chicken is no longer pink, about 6-8 minutes.

In a large mixing bowl, combine the cooked chicken and onions with the orzo, chicken broth, sour cream, and half of the cheddar cheese. Stir until well mixed.

Transfer the mixture into a greased 9x13 inch casserole dish and spread evenly. Top with the remaining cheddar cheese.

Cover the dish with aluminum foil and bake for 25 minutes.

Remove the foil and bake for an additional 10-15 minutes or until the cheese is bubbly and golden brown.

Let it cool for 5-10 minutes before serving.