Place the flour in a shallow bowl. Dredge both sides of each chicken breast in the flour so they are lightly coated.
In a large, non-stick frying pan, melt the butter. Add the chicken. Cook for 2 - 3 minutes per side, or until the chicken begins to brown. Remove the chicken to a clean plate.
Meanwhile to the same pan, still over medium heat, add the shallots and garlic. Cook for 1-2 minutes, stirring often until fragrant.
Add the mustard, sage, salt and pepper. Stir to combine.
Pour in the apple cider and whisk until smooth. Bring to a boil. Then reduce heat so the glaze is simmering.
Return the chicken to the pan with the apple cider glaze. Cover the pan and simmer the chicken in the sauce for about 10 minutes. Check the temperature of each chicken breast. They need to be a minimum of 165 degrees F for safe eating.
Place the cooked chicken into a clean bowl. Continue to simmer the apple cider glaze if you'd like it to be a little thicker. Then pour it over the chicken to serve.