Sprinkle the drumsticks with salt and pepper. Heat the oil in an oven-safe cast iron frypan and fry the drumsticks until browned, about 10 minutes.
Meanwhile, put the potatoes in a saucepan of cold water and bring to the boil. Drain and set aside.
Scatter the potatoes and mushrooms around the chicken. Add the garlic cloves and rosemary, then bake 30 minutes in the preheated oven.
Place the frypan back on the stovetop and add the chicken stock. Bring to the boil, then remove the chicken to a hot plate. Add the lemon juice to the sauce, pour the sauce over the drumsticks and sprinkle with parsley.