1) Line a 9" pie pan with the pastry, and refrigerate while you make the filling. Preheat the oven to 425°F.
2) Mix the sugar, thickener, and salt. Toss with the peaches, extracts, and lemon juice. Spoon into the crust.
3) Make the topping by combining the brown sugar, flour, oats, and nuts, then mixing in the soft butter until the mixture is crumbly. Sprinkle the topping over the filling.
4) Place the pie on a parchment-lined baking sheet. Bake for 20 minutes, reduce the oven heat to 350°F, and bake for 40 minutes more, until the filling bubbles. Cover the edges of the pie with foil or a pie shield if they brown too quickly. You can also shield the streusel with foil if it's over-browning.
5) Remove the pie from the oven, and cool it completely before slicing.
Yield: one 9? pie, 8 to 10 servings.