Melt butter in heavy large skillet over medium heat. Add onion and sauté until tender, about 8 minutes. Mix in spinach and sauté until wilted, about 2 minutes. Add zucchini and continue to cook until almost all liquid from spinach evaporates, about 3-4 minutes.Set aside to cool slightly.
In a large bowl, combine ricotta, mozzarella and Parmesan cheeses. Mix in eggs. Add cooled spinach mixture; blend well.
Spoon cheese mixture into pie crust and smooth the top. Bake until filling is set in center and brown on top, about 40 minutes. Let stand 10 minutes. Cut pie into wedges.