Cheesy parsnip and sweet potato tarts

Name: Cheesy parsnip and sweet potato tarts
Ingredients: 225g/8oz plain flour, plus extra for dusting; pinch salt; 150g/5½oz cold butter, cut into cubes; 50g/1¾oz Parmesan (or alternative vegetarian hard cheese), finely grated; 2½-3½ tbsp iced water; 10g butter; 1 onion, finely chopped; 2 parsnips, peeled and cut into 1cm cubes; 1 sweet potato, peeled and cut into 1cm cubes; 2 free-range eggs; 200ml/7fl oz double cream; 2 sage leaves, finely chopped; 1 tsp fresh thyme leaves, plus 12 thyme sprigs to garnish; 60g/2¼oz Parmesan (or alternative vegetarian hard cheese), finely grated; 30g/1oz blanched hazelnuts, roughly chopped; salt and freshly ground black pepper