Defrost the phyllo dough according to package directions.
Wash spinach. In a colander, start salting and rubbing the salt into the spinach. Tear and squeeze the spinach until its size is greatly reduced. When you have as much water out of the spinach as possible, rinse it well to remove the salt. Then you will have to squeeze it again to remove water absorbed during rinsing.
Saute the onion in the olive oil.
Crack eggs into a large bowl. Beat them. Add the feta, oregano, spinach and onion. Grind in some black pepper.
Melt the butter.
Brush a 9x13” pan with butter. Lay out the phyllo dough on the counter by the pan. Have melted butter in a bowl nearby.
Lay individual layers of phyllo into the pan, brushing in between. Lay some layers from each side; may have to fold some layers in half on the short side. Work quickly so the phyllo does not dry out.
Pour the spinach filling into the center of the phyllo.
Fold the phyllo layers back over the filling, again brushing butter on every layer. Lay a smooth folded sheet on top.
