Preheat the oven to 400 degrees F.
Line a heavy large baking sheet with parchment paper. Arrange the frozen pastry shells on the baking sheet and bake until they puff and become golden brown, about 20 minutes. Using a small knife, cut out the center of the pastry shells.
Meanwhile, stir the cream, milk, salt, and pepper in a medium bowl. Stir in the spinach, Fontina cheese, Parmesan, and green onions. Spoon the spinach mixture into the baked pastry shells. Bake until the pastries are golden brown and the filling is hot, about 25 to 30 minutes. Transfer the pastries to a platter and serve.