Meanwhile, heat oil in a large skillet over medium heat. Add leeks and cook, stirring, until lightly browned, 6 to 8 minutes. Add mushrooms and 1/2 teaspoon rosemary; cook, stirring, until softened, about 5 minutes. Add flour and cook, stirring, for 1 minute. Add broth and milk; bring to a boil, stirring, until slightly thickened, about 4 minutes more. Remove from heat and stir in chicken. Season with lemon juice, salt and pepper.