In a nonstick saucepan, sauté chicken breast halves in the olive oil for 7 minutes on each side, or until golden brown. Place the cooked breasts on a plate and add the butter to the remaining oil. Slice the zucchini into 1/4- inch-thick pieces and sauté for 5 minutes. Add the cooked zucchini to the chicken on the plate. Remove any excess oil from the pan. Over high heat, add the wine and vinegar to the pan and stir the mixture with a spoon until it's thick and syrupy. Add the cream and boil vigorously, continuously scraping the pan with the spoon until the cream is thick and reduced to about half its original volume. Remove from heat, stir in the mustard and spoon the mixture over the chicken and zucchini.