Preheat the oven to 200°C / 180°C fan / 400°F / gas mark 6
Top and tail the leek and remove the outer leaves. Slice into 4cm pieces (on a diagonal is best).
Place the leeks in a small casserole dish and spray with most of the calorie controlled cooking oil. Add the white wine vinegar and the water and stir.
Next wrap each chicken breast with one slice of proscuitto.
Place the wrapped chicken on top of the leeks and top with a sprig of thyme.
Spray with the remaining calorie controlled cooking oil.
Cook in the centre of the preheated oven for 25 to 30 minutes or until the chicken is thoroughly cooked through.