Turn the heat under the skillet to medium-high, and simmer the mushroom sauce for about 4 minutes until it is reduced by about a third.
Stir in the 1/2 cup heavy cream into the mushroom sauce and simmer for another 2 minutes on low-medium heat, constantly stirring and scraping from the bottom of the skillet, until the sauce gets thicker.
Add chicken legs back to the skillet to warm them up.
