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.