In a large pot, melt your butter on medium heat and saute the onions, carrots, celery until soften, but not brown. Stir in the garlic for 30 seconds.
Sprinkle over the flour and mix well to form 'paste'.
Gradually add in 2 cups of the broth and the milk, while stirring constantly so the mixture stays thick. Add in the remaining 3 cups broth, orzo, seasonings and turkey pieces. Bring back to a boil then reduce the heat to a simmer. Half cover the pot and allow to cook for another 10 minutes (stirring occasionally), or until the pasta is cooked al dente.