Mix the whole wheat flour with the salt and rub in the cold butter with your fingertips until sandy. Add the egg and cold water and bring together into a dough, then wrap and chill for 15 minutes.

Roll out the dough and line a tart tin, prick with a fork, weigh it down with baking paper and dried beans, and blind-bake in an oven preheated to 190°C for 8 minutes.

Soften the onion in olive oil, add the spinach and cook until it wilts and releases its water. Whisk the eggs with the milk, stir in the white cheese and black pepper, then fold in the spinach mixture.

Pour the filling into the pre-baked crust, scatter over the grated kashar cheese and bake for 12 more minutes, until the centre has just a slight wobble.