Add the remaining 1 tablespoon oil and onion to the pan. Cover and cook, stirring occasionally, until golden, about 3 minutes. Stir in wine and mustard, scraping up any browned bits. Reduce heat to maintain a simmer. Stir in cream and capers. Return the chicken and any accumulated juices to the pan. Cover and simmer until an instant-read thermometer inserted into the thickest part of a thigh registers 165°F, about 4 minutes. Serve topped with parsley, if desired.