Cook the potatoes in a saucepan of boiling water for 12–15 minutes until tender. Drain, then cut into bite-sized pieces.
Make a slit lengthways down the side of each chicken breast to form a pocket, ensuring that you do not cut all the way through. Mix together the herbs, garlic and crème fraîche, season well with pepper, then spoon a little into each chicken pocket.
Put the leeks, chicory and potatoes in an ovenproof dish. Pour over the stock, then lay the chicken breasts on top. Spoon over the remaining crème fraîche mixture, then bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 25–30 minutes until the chicken is cooked through and the vegetables are tender.