Combine the flour, sugar and a pinch of salt in a large bowl. Mix in the cold butter with a pastry blender or your hands until the mixture resembles coarse crumbs. Work in egg yolks and 1 to 2 tablespoons ice water until the dough holds together without being too sticky. If it's still crumbly, add more ice water, 1 teaspoon at a time. Form into a ball, wrap in plastic and refrigerate for at least 30 minutes.
On a floured surface, roll the dough into an 11 to 12-inch circle with a floured rolling pin. Ease the dough into a 9-inch pie pan, pressing firmly into the bottom and sides. Trim the excess dough, then pinch the edges. Refrigerate until ready to fill.
Preheat the oven to 350 degrees F.
Split the pumpkins down the middle and scoop out the seeds. Arrange on a roasting tray, cut side up, and roast for 20 minutes. Remove the pumpkins from the oven and lay bananas (in their skins) around and between the pumpkin halves. Return to the oven and roast for another 10 minutes. Remove from oven and let cool slightly before handling, about 5 minutes.
Scoop out the pumpkin flesh and add to a food processor. Peel the bananas and add the flesh to food processor as well. Add the 3 eggs, sugar, cinnamon, 2 tablespoons butter, nutmeg, heavy cream and salt. Puree until smooth.
Put a baking sheet on the bottom oven rack and preheat to 375 degrees F.
Pour the filing into the prepared pie shell. Lightly beat the egg white. Brush the pastry edges with the egg white and sprinkle with a bit of sugar. Carefully arrange the pie on the preheated baking sheet, lower the oven temperature to 325 degrees F and bake it until the pie is set but still jiggles slightly, about 50 minutes. Remove from the oven and let cool completely.
Beat the egg whites, powdered sugar, cream of tartar, and a pinch salt in a large bowl with an electric mixer until glossy medium peaks form, being careful not to over beat.
Spread the meringue mixture over the top of the pie, covering right to the edges. Using the back of a spoon press lightly into the meringue and pull up on the spoon to form peaks all over the surface of the meringue (alternatively you can use a piping bag with a large star tip if you choose).
To cook the meringue, preheat the broiler with the rack in the highest position. Put the pie under the broiler and broil until the meringue is set and golden brown, about 7 minutes, watching carefully to avoid burning, as it cooks quickly. Remove from the broiler and cool before serving.