Melt butter in a large skillet over medium heat.
Add the onions, bell peppers, and mushrooms.
Cook for about 5 minutes, until soft.
Sprinkle flour over the vegetables.
Stir constantly for 1-2 minutes to cook the flour.
Gradually whisk in milk and chicken broth.
Continue stirring until the mixture is smooth and thickened.
Stir in the cooked chicken, salt, pepper, and garlic powder.
Reduce heat to low and simmer for another 10 minutes.
Serve hot over rice or toast for an enjoyable meal.