1Season chicken on both sides with salt and pepper. Heat 1½ teaspoons oil in a large heavy skillet over medium-high heat. Add chicken and sear until well browned on both sides, about 3 minutes per side. Transfer the chicken to a plate and tent with foil.
2Reduce heat to medium. Add the remaining 1½ teaspoons oil to the pan. Add leek and thyme. Cook, stirring, for 2 minutes. Add mushrooms; cook for 5 minutes more. Add brandy and cook for 2 minutes. Transfer to a bowl. Whisk broth and flour in a small bowl. Add to the pan. Cook, whisking, until slightly thickened, about 3 minutes.
3Return the mushroom mixture, chicken and any accumulated juices to the pan; reduce heat to low. Simmer until chicken is cooked through, about 4 minutes. Transfer the chicken to a warmed platter. Season sauce with salt, pepper and lemon juice; spoon over the chicken. Garnish with parsley.