In a large high sided skillet, melt the butter over medium heat.
Add the onion and mushrooms, season with salt, pepper, paprika, and garlic powder if using and sauté until softened (5 minutes).
Sprinkle the flour, and cook it while stirring with a spoon for 1 minute. Reduce the heat to low.
Slowly pour in the chicken stock, and use a whisk to stir it until smooth. Then add in the milk, and keep stirring with a whisk until you get a smooth creamy sauce. Allow to thicken for a couple of minutes.
Stir in the chicken, peas, and pimentos. If the sauce is too thick, add a splash of water and stir again.
Remove from heat, and serve over pasta, rice, or biscuits. Garnish with chopped fresh parsley.