Preheat oven to 350°F. Unfold pie crust. Press out fold lines; if crust cracks, wet fingers and push edges together to seal. Sprinkle flour over crust. Place crust floured side down in 9-inch-diameter glass or ceramic pie dish. Fold edge under and crimp decoratively.
Melt butter in heavy large skillet over medium heat. Add onion and sauté until tender, about 8 minutes. Mix in spinach, salt, pepper and nutmeg. Sauté until all liquid from spinach evaporates, about 3 minutes.
Combine ricotta, mozzarella and Parmesan cheeses in large bowl. Mix in eggs. Add spinach mixture; blend well.
Spoon cheese mixture into pie crust. Bake until filling is set in center and brown on top, about 40 minutes. Let stand 10 minutes. Cut pie into wedges.
