Place washed spinach leaves in a large bowl. Sprinkle very heavily with salt and work salt into leaves with hands while tearing leaves into small pieces (use gloves if you have sensitive skin or if you have any cuts!). When spinach has been reduced to about 1/4 of the original volume, rinse thoroughly to remove all the salt and drain well.
In another bowl, beat eggs and mix in the feta. Add to spinach.
In a nonstick skillet, heat a small amount of olive oil and saute onions until just starting to brown. Add to spinach mixture.
Season spinach mixture with pepper to taste (you probably won't want salt, as there will be some residual salt in the spinach and the feta is quite salty). Mix in a little oregano (I use a couple of generous pinches of dried).
Brush a 9x13 inch baking dish with butter or spray with olive oil spray. Heat oven to 375* F.
Place the filo on a flat working surface and keep covered with slightly dampened paper towels while working.
Take 1 sheet at a time, brush with butter or spray with olive oil spray, and place in the pan so that edges hang over sides. Continue until all but a few sheets have been used, fanning the sheets out so that you end up with the bottom and sides of the dish all covered and an even amount of overhand all around the dish.
Pour the filling into the pan. Fold the overhanging edges of filo to cover most of the top. Brush/spray the remaining few sheets of filo, place on top, and tuck in around sides of pan. Cut 3 slashes in the top with a very sharp knife.
Brush top with butter or spray with olive oil spray, place in hot oven, and bake for 50 minutes or until hot and golden brown.
Cut into squares and serve while still very hot.