Perfect cauliflower cheese with bacon

Name: Perfect cauliflower cheese with bacon
Prep time: 30 min
Cook time: 1 h
Total time: 1 h 30 min
Cuisine: British
Ingredients: 1 head cauliflower, trimmed and broken into florets; 250g/9oz smoked streaky bacon, cut into 1cm/½in strips; 250g/9oz chestnut mushrooms, finely sliced; 25g/1oz butter; 50g/2oz plain flour; 250ml/9fl oz full-fat milk; pinch English mustard powder; 200g/7oz gruyère cheese, grated; 55ml/2fl oz double cream; salt and freshly ground black pepper; pinch ground nutmeg; 50g/2oz ciabatta breadcrumbs; 50g/2oz parmesan cheese, finely grated; green salad; crusty bread