1) To make the crust: In a large bowl, whisk together the flour, buttermilk powder, and salt.
2) Mix in the cold butter and shortening until the dough is crumbly.
3) Sprinkle ice water over the mixture 1 tablespoon at a time while tossing with a fork. After adding 3 tablespoons of water, gather the dough together into a ball. Add more water if it crumbles and won't hold together.
4) Flatten into a disk, wrap, and refrigerate for 30 minutes or longer.
5) Preheat the oven to 450°F.
6) Whisk together the sugar, Instant ClearJel, ascorbic acid, nutmeg, and salt.
7) Add the peaches and a few of the raspberries, tossing to combine. Set aside.
8) Roll the chilled dough on a well-floured work surface or silicone rolling mat into a 14" circle.
9) Use a knife or pizza wheel to trim the edges in a scalloped design.
10) Move the crust to a baking pan or pie pan; a giant spatula works well here.
11) Fill the center with the peach mixture.
12) Fold the edges of the crust up over the peaches, leaving the center uncovered.
13) Bake the tart for 40 to 45 minutes, until the crust is golden.
14) Remove from the oven, and sprinkle with the remaining fresh raspberries.
15) Serve warm, with whipped cream or ice cream.