01 - Preheat the oven to 190°C (375°F).
02 - In a bowl, mix the flour and salt. Add the cold butter and cut it into the flour until the mixture resembles coarse crumbs. Stir in the ice water, a tablespoon at a time, until the dough comes together.
03 - Roll out the pastry and fit it into a 23 cm (9-inch) tart pan. Prick the bottom with a fork and chill for 15 minutes.
04 - Heat olive oil in a pan over medium heat. Sauté the spring onions until soft, about 5 minutes. Remove from heat and let cool.
05 - In a large bowl, whisk together the eggs, sour cream, milk, salt, and pepper. Stir in the sautéed spring onions and grated cheese.
06 - Pour the filling into the prepared pastry shell and bake for 30–35 minutes, or until the filling is set and the top is golden.
07 - Let the tart cool slightly before slicing and serving.