The perfect potato gratin

Name: The perfect potato gratin
Cook time: 1 h 20 min
Total time: 1 h 20 min
Cuisine: French
Ingredients: 1 small knob butter; 200 ml semi-skimmed milk; 300 ml double cream; 2 bay leaves; 2 cloves garlic peeled and finely sliced; sea salt; freshly ground black pepper; 2.5 kg potatoes peeled and thinly sliced; 1 handful fresh thyme; 1 small handful Parmesan cheese freshly grated; olive oil; 6 rashers higher-welfare streaky bacon chopped; 1 handful vac-packed chestnuts peeled and crumbled