Heat oven to 325 degrees. Combine cracker crumbs and 1 stick melted butter in a small bowl. Transfer to a greased 9-inch pie dish; press evenly into bottom and sides of dish. Set aside.
Heat remaining 1 T of butter and olive oil in a large skillet over medium heat. Add onion; cook until softened, about 2 minutes. Add spinach; cook until tender, about 3 minutes. Spread mixture over crumb crust. Set aside.
Beat together eggs and milk in a medium bowl. Mix in cheese and salt. Pour mixture into crumb crust. Bake until pie is set and golden brown, about 50 minutes.