Heat 2 tablespoons of olive oil in a large sauce pan. Add in the mushrooms and brown them until golden. See Notes about sautéing mushrooms. Once the mushrooms are golden, remove them from the pan and set them aside in a small bowl to add back later.

Add the celery, carrot and onion to the pan and sauté the vegetables until they are soft. About 5 minutes. Add the minced garlic and continue to cook for a further 2-3 minutes.

Stir the rice into the vegetables to coat with some of the oil. The rice will get slightly toasted and give a depth of flavour.

Pour the chicken broth to the pot and simmer for about 10 minutes until the rice is almost cooked through. Add the diced chicken and continue to simmer for another 10 minutes until the rice is fully cooked and the chicken is cooked through. Set the pot of soup aside while you make the roux sauce.

In a medium sauce pan over medium heat melt the butter. Once the butter is melted add the plain flour. Stir together until a thick paste forms. Slowly add the milk and whisk continuously until the sauce is thick and smooth. Bring to a boil. See notes. Once the roux sauce has boiled season with salt and pepper. Gently pour the thick roux sauce into the chicken rice soup and add back the mushrooms. Stir well.