Melt the butter in a shallow, flameproof casserole dish. Add the leeks and cook for 3 minutes until softened. Add the chicken and cook for 2 minutes until lightly browned all over. Stir in the ham, stock and crème fraîche, then season to taste.
Mix the flour and baking powder in a bowl, then pour in the oil and milk. Mix gently, season well and stir in the herbs and cheese.
Arrange spoonfuls of the dough on top of the chicken mixture, leaving a little space between each spoonful. Place in a preheated oven, 220°C (425°F), Gas Mark 7, for 15–20 minutes until the topping is lightly browned and the chicken is cooked through.