First, preheat oven to 375ºF and spray a casserole dish with nonstick cooking spray.
Next place 1.5 cups uncooked quinoa on the bottom of the casserole dish.
Then, finely dice onion and cut chicken breast into bite-sized pieces. Transfer into casserole dish.
Season chicken with minced garlic, Italian seasoning, garlic powder, salt, and pepper.
Next, mix together around 3 cups of tomato soup with 1 cup of water. If you buy tomato soup in a carton, you will get anywhere from 3-4 cups of tomato soup, so you can use all of the soup, but substitute the rest with water.
Pour soup mixture on top of chicken and quinoa and use a spoon to mix, making sure that the chicken and quinoa are submerged.
Cover casserole dish with tin foil and bake at 375ºF for 30 minutes.
While the casserole is baking, slice 5-6 medium tomatoes.
Once the 30 minutes is up, remove the casserole from the oven and stir. Add sliced tomatoes and bake for an additional 20 minutes, uncovered.
Finally, remove casserole one more time, add mozzarella cheese and bake for an additional 10 minutes.
Remove casserole dish from oven and let sit for 10 additional minutes to thicken.
Serve with fresh basil.