In a food processor, combine the flour with the sugar, salt and baking powder and pulse to blend. Add the butter and pulse until the mixture resembles coarse meal. Sprinkle the ice water over the mixture and pulse a few times, just until a dough starts to form. Scrape the dough onto a lightly floured work surface and gently knead it a few times. Pat the dough into a disk, wrap in plastic and refrigerate until chilled and firm, at least 2 hours or overnight.
In a bowl, combine the brown sugar with the flour, almonds, oats, cinnamon and salt. Add the butter and with a pastry cutter or your hands, cut or rub the butter into the mixture evenly. Refrigerate the streusel.
Preheat the oven to 350°. On a lightly floured work surface, roll out the dough to a 14-inch round, about 1/8 inch thick. Fold the dough in half and transfer it to a 9-inch glass pie plate. Unfold the dough and gently press it into the plate. Trim the overhanging dough to 1/2 inch and fold it under itself; crimp decoratively. Freeze the pie shell for 10 minutes.
In a large bowl, toss the sliced fruit with the sugar and lemon juice and let stand for 5 minutes to let the sugar dissolve. Stir in the cornstarch. Scrape the fruit into the pie shell in an even layer. Scatter the streusel evenly on top of the fruit.
Bake the fruit pie in the lower third of the oven for about 1 hour and 15 minutes, until the fruit starts to bubble around the sides. Transfer the pie to a wire rack and let cool to room temperature. Cut into wedges and serve with vanilla ice cream.