Preheat the oven to 220C/425F/Gas 7.
For the chicken, use a sharp knife to cut a pocket lengthways into the chicken breast. Mix the cheese and rosemary together in a bowl, then stuff the mixture into the pocket of the chicken breast. Wrap the chicken in the slices of bacon to cover and seal the pocket.
Heat the butter in an ovenproof frying pan over a high heat. Add the chicken and fry for 1-2 minutes on each side, until the bacon is golden.
Place the frying pan into the oven and roast for 15 minutes or until the chicken is completely cooked through.
Meanwhile, for the leeks, heat the butter and the oil in a frying pan over a medium heat. Add the garlic, onion and leek, and sauté for 2-3 minutes, until soft.
Add the double cream to the frying pan and season, to taste, with salt and freshly ground black pepper.
To serve, place the leeks onto a serving plate and top with the chicken breast.