Heat oil in large skillet over medium-high heat. Add onion and saute until translucent and just golden, about 8 minutes. Remove from heat, transfer to a large bowl and allow to cool while preparing crust.
Remove 1 of the prepared pie crusts from the refrigerator, and place in an ungreased, 9-inch pie plate. Shape the crust to fit the form of the pan with a 1/2-inch overlap around the edge.
To the onion mixture, add the ham, spinach, cheeses, 4 eggs plus 1 yolk, salt, and pepper, and mix well. Spread mixture into prepared crust. Cover with remaining pie crust; crimp edges and score to vent. Brush with egg white and bake until golden brown, about 40 to 45 minutes. Let stand at least 15 minutes and serve warm or at room temperature.