In a large bowl (or food processor), combine the plain flour, wholemeal spelt flour, and salt.
Add the chilled butter and rub in with your fingertips (or pulse in the processor) until the mixture resembles fine breadcrumbs.
Add the cold water, mixing until the dough just comes together.
Bring the dough into a ball, flatten into a disc, wrap in cling film, and chill in the fridge for at least 30 minutes.
Roll out the chilled pastry on a lightly floured surface to about 2–3mm thick. (Or roll out on a large piece of parchment paper, then flip it over to line the case).
Line a 24cm tart tin with the pastry, pressing it gently into the corners and up the sides. Trim, leaving a slight overhang. Chill again for 20 minutes.
Preheat the oven to 180°C (fan 160°C) / 350°F / Gas Mark 4.
Line the pastry with baking parchment and fill with baking beans or rice.
Bake for 15 minutes, remove the baking beans and paper, then return to the oven for 8–10 minutes until the base is dry and lightly golden.