The perfect potato gratin

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