Place a large skillet over medium-high heat. When pan is hot, add butter and melt. Add chicken breasts and cook for 6 minutes. Turn chicken over and cook for another 6 minutes. Remove chicken from skillet and transfer to a plate. Cover with aluminum foil to keep warm.
Add onions to skillet and cook for 5 to 7 minutes, scraping any bits off the bottom of the pan. Add cream and vinegar. Bring mixture to a boil, reduce heat, and simmer for 4 minutes. Stir in mustard and parsley. Season with salt and pepper. Spoon sauce over chicken and serve.
Go to full version of