Reduce heat to medium. Add mushrooms and cook, stirring, for 1 minute. Add spinach, cover and cook, stirring once, until the mushrooms are tender, 2 more minutes. Whisk cream, sour cream, vinegar, garlic powder, onion powder and the remaining 1/4 teaspoon each salt and pepper together in a measuring cup. Add to the pan along with any accumulated juices from the chicken; simmer, stirring, until slightly thickened, 1 to 2 minutes. Return the chicken to the pan and turn to coat. Serve the chicken topped with the sauce.