Heat a large nonstick skillet over medium-high heat. Cook the flour for 2 minutes, or until light golden brown, stirring constantly. Transfer to a small bowl.
Reduce heat to medium. Add oil to the skillet and swirl to coat bottom. Cook the chicken smooth side down for 3 minutes. Turn and cook for 3 more minutes or until no longer pink. Transfer to serving plate and cover with foil to keep warm.
Meanwhile, whisk 1/4 cup of broth into bowl with browned flour until completely blended. Whisk in the remaining broth, milk, garlic powder, salt, and mustard. Pour into skillet, scraping with a falt spatula to dislodge any browned bits. Increase heat to medium high and bring to a boil. Cook for 4 minutes, or until reduced to about 1/2 cup, stirring frequently. Remove from the heat. Stir into the margarine. Spoon over chicken. Sprinkle with the green onions.