Prepare the crust: Put the flour and salt in the bowl of a food processor.
Cut the butter into chunks and add to the flour.
Process in on/off pulses until coarse crumbs are formed.
In a small bowl beat together the egg yolk and yogurt and add to the flour mixture while the machine is running.
Remove and shape into a neat ball. (The dough should not be sticky. If it is, knead it briefly on a floured surface.).
Flatten the ball and using thumbs, press into a 10 inch pie plate, distributing the dough around equally, with no dough build-up around the inner edges. Flute the top.
Use a fork to pierce the bottom and sides of the dough, so it will not rise, or weigh the dough down with beans placed on a circle of parchment paper for easy removal. Bake the crust for 10 minute, or until just lightly browned.
Remove from the oven and let cool slightly. Remove the beans and the parchment paper, if using.
Thaw the spinach and put in a mesh strainer to drain. Press and squeeze out any excess liquids.
In a food processor, chop the scallions and add the rest of the ingredients except the spinach and process briefly in on/off pulses.
Pour the filling into the partially-baked crust and bake 10 minutes.
Lower heat to 175C and bake an additional 10-15 minutes or until the quiche is set, and a toothpick inserted in the center comes out moist but not wet.