Make the crust: In a food processor, pulse flour, wheat germ, sugar, salt, and baking powder. Add butter and process until the largest pieces are the size of peas. Add buttermilk and pulse until dough just holds together. Wrap in plastic and pat into a rectangle. Refrigerate for 1 hour, or up to overnight.
Make the filling: Preheat oven to 450 degrees. On a rimmed baking sheet, toss squash with oil and 2 tablespoons brown sugar. Roast until golden and tender, about 15 minutes. Remove from oven and reduce temperature to 400 degrees.
In a bowl, stir together remaining 1/4 cup brown sugar, ground almonds, salt, egg, vanilla, and flour.
Roll out dough on lightly floured parchment paper to a 9-by-15-inch rectangle. Spread almond mixture on top, leaving a two-inch margin around the edge. Top with squash. Fold dough over each side. Lightly brush crust with water and sprinkle with raw sugar. Chill until firm, about 30 minutes.
Bake on a rimmed baking sheet until crust is golden brown, 25 to 30 minutes. Sprinkle with sliced almonds.
Dough can be frozen for up to 3 months.