Preheat oven to 350 degrees. Cook pasta according to pkg directions. Drain pasta. Set aside.

Melt butter in large skillet over medium-high heat. Add mushrooms & onion powder. Stir & cook for 2 minutes. Add chicken broth & 1/2 cup milk to skillet. Stir until blended.

Add flour to remaining 1/2 cup milk. Whisk until well blended. Slowly stir mixture into skillet.

Bring mixture in skillet to a boil. Cook for 1 minute. Reduce heat to low. Add 2 tbsp parmesan & stir. Remove from heat. Stir in sour cream until well blended. Add chicken & pasta. Mix well.

Lightly spray 1 1/2 qt casserole dish with cooking spray. Transfer chicken mixture to prepared casserole dish. Sprinkle with remaining 2 tbsp parmesan.

Bake until heated through, about 30-35 minutes.