Quick Chicken Fricassee

"In search of a streamlined technique that would give this classic French braise weeknight potential and a brighter, more complex sauce, we replaced the bone-in chicken parts with convenient boneless, skinless breasts and thighs. We found two ways to add back the richness that we’d lost when we opted for boneless chicken: We browned the meat in a combination of butter and oil, and we browned the vegetables until they developed their own fond to serve as the base of the sauce. Increasing the amount of glutamate-rich mushrooms boosted the fricassee’s meaty flavor. As a final step, we finished the sauce with sour cream, which added body and a pleasant tang. Whisking an egg yolk into the sour cream thickened the sauce and made it incredibly silky...."