Return pan to medium-high heat. Add remaining 2 teaspoons oil. Add potatoes, cut sides down; mushrooms; and 1 tablespoon thyme; cook 3 minutes or until browned, stirring once. Combine milk and flour in a small bowl, stirring with a whisk. Add remaining salt, remaining pepper, flour mixture, stock, lemon, and beans to pan; simmer 1 minute or until slightly thickened. Add chicken; cover, reduce heat, and simmer 3 minutes or until beans are crisp-tender. Sprinkle with parsley.