Measure out the flour and salt in a bowl, cut in the butter then mix in the olive oil and water. Add a little extra water or flour as needed to have it sticking together but not wet or too flaky. Alternatively mix together with a mixer/food processor adding the water last a tablespoon at a time.
Roll out the pastry on a lightly floured surface and fill a tart/quiche pan which is approx 9.25in/23.5cm diameter and 1in/2.5cm deep. Press into the corners then trim around the edges. Prick all over with a fork. (Skip the above steps if using a ready made pastry case.)
Blind bake the pastry case for 10 minutes, remove from the oven, take out the parchment/foil from blind baking and let it cool slightly.
Meanwhile, cut the smoked salmon in to thin strips. Whisk together the eggs, sour cream, milk and a little black pepper, to taste.
Spread the smoked salmon out evenly across the bottom of the pastry case. Top with the chopped chives then carefully pour in the egg-cream mixture.
Bake for approx 25 minutes until lightly golden and set (depending on your oven, you may want to turn it a little over halfway through). Serve warm or room temperature.
I often find it best to hold back a little of the egg mixture until I transfer it to the oven and top it up while it's on the chef.