Coarsely chop spinach leaves. In a food processor or blender, whirl spinach, green onions, eggs, half-and-half, cheese, flour, nutmeg, 1/4 teaspoon salt, and pepper until smooth.
Line an oiled 9-inch pie pan smoothly with foil, overlapping pan rim; oil foil. Cover foil with prosciutto, slices overlapping so there are no gaps. Slowly pour in spinach mixture. Gently lay pastry round on filling (it's soupy; avoid getting filling on top of pastry). Fold excess pastry under itself flush with pan rim; press around edges with a fork.
Bake in a 350° oven until crust is golden brown and a slender wood skewer inserted into center comes out clean, 40 to 45 minutes (30 to 35 minutes in a convection oven). Cool on a rack at least 20 minutes. Invert a plate over torta; holding pan and plate together, invert to release torta onto plate. Peel off foil. Serve warm or at room temperature, cut into wedges.