1/4 cup apple brandy, such as Calvados or Apple Jack
3 tablespoons unsalted butter
2 tablespoons flour
1/2 cup sour cream
3 tablespoons minced green onion
In a large skillet over medium heat, bring the apple juice to a simmer. Add the apples and leeks; simmer, stirring occasionally, for about 3 to 5 minutes, or until softened. With a slotted spoon, transfer apples and leeks to a bowl; set aside.
Increase the heat to high and bring the remaining apple juice to a boil.
Cook until reduced to about 1/4 cup, about 5 minutes. Pour reduced juice into a small bowl and set aside.
Rinse the chicken and pat dry. Sprinkle all over with salt and pepper. Arrange in a single layer in the skillet. Pour chicken broth and apple brandy over chicken. Place skillet over medium-low heat and cook, covered lightly so steam can escape, about 10 to 14 minutes, until chicken is cooked through and juices run clear. Remove chicken to a plate with slotted spoon.
Increase the heat to high and boil the liquid until reduced to 1 1/2 cups, about 6 to 8 minutes. Pour into the bowl with reduced apple juice.
Add the butter to the skillet and heat over medium heat until melted. Whisk in flour and cook, stirring, 1 to 2 minutes, until smooth and bubbly. Whisk in the reserved juices and bring to boil, whisking until thickened, about 3 to 5 minutes.
Reduce heat to low; stir sour cream into the thickened sauce. Add the leeks, apples, and chicken to the sauce, gently stirring to coat well. Cook about 2 minutes, just until heated through.