PREHEAT oven to 400F. Squeeze spinach well to remove excess moisture. It should measure about 2 cups.
BEAT 2 eggs with 1 egg white and cream cheese in a medium bowl, until combined.
HEAT a medium (10-in.) oven-safe frying pan over medium. Add oil, then onion. Cook until onion is soft, about 3 min. Stir in spinach, feta, dill, salt and nutmeg. Remove from heat. Stir in egg mixture until evenly distributed.
CUT out holes all over pastry with the large end of a piping tip, or a paring knife, so steam can escape during baking. Cover filling with pastry, stretching dough to the edges of pan. Tuck in corners.
BAKE in centre of oven until crust is lightly browned, 20 min min. Beat reserved egg yolk and brush over pastry. Continue baking until golden brown, about 5 more min.