Heat oil in a large, high-sided, ovenproof skillet over medium-high heat. Add chicken and chopped onion; cook, stirring occasionally, until the chicken is no longer pink on the outside, about 7 minutes. Sprinkle the mixture with flour and cook, stirring occasionally, for 1 minute. Add milk to the pan and bring to a boil, stirring frequently. (Be careful, the pan will be very full.) Boil, stirring, for 1 minute.