Heat oven to 200C/180C fan/gas 6. Roll out the pastry on a lightly floured surface and line a deep 12-inch tart tin. Cover with baking parchment and fill with baking beans (or rice if you don't have baking beans). Cook for 20 mins, then remove the paper and beans and cook for 5-10 mins more until the tart case is pale
Turn down the oven to 160C/140C fan/gas 2. Position the potatoes then spring onions and cheddar on the pastry case.
Whisk together the eggs, milk and cream, then season and pour over the pastry. Sprinkle a handful of spring onions on the top and pop on the middle shelf of the oven.
Cook for 40-45 mins or until just set. Remove, cool on a wire rack then serve.