1Heat oven to 350 degrees. Prepare pie crust according to directions on the box.
2Saute onion in whole stick of butter, until translucent. In the meantime, squeeze out all remaining water from the spinach. Add the spinach to the onions, just long enough to cook off any excess liquid. Set aside.
3Grease an 13x 11 inch pan and sides with canola oil spray. Roll out a little more than half of the pie dough and place on bottom and sides of pan.
4Beat eggs and add remaining ingredients. Combine this with the spinach and onions, and place in pie crust. Cover with remaining pie dough. Combine reserved egg yolk with a little oil and brush on crust. With a sharp knife, score the top of the dough to mark where you'd like to later cut and serve the pieces.