Preheat the oven to 375°. Line a 9x12-inch baking pan with parchment paper (you want the parchment to come up the sides so you can use it as "handles" later to lift the dessert out of the pan).
With an electric mixer, beat the butter with the sugar at medium speed until light and fluffy, about 2 minutes. At low speed, beat in the flour, spices and salt until a soft dough forms.
Use clean hands to press the dough over the bottom of the prepared pan and ½ inch up the side in an even layer. Bake in the center of the oven for about 15 minutes, or until the crust is golden and set. Let cool on a rack.
FILLING
Place the peaches in a large bowl. Drizzle in the butter and stir in the sugar, cinnamon and nutmeg.
TOPPING
In a large bowl, mix the oats with the flour, light brown sugar, cinnamon, baking soda and salt.
Using fingers (wash your hands and take off your rings first!), squeeze the butter into the oats & sugar and crumble together until the mixture resembles coarse meal. (Alternately, you can use a pastry cutter, but ultimately you'll want to just use your hands).
ASSEMBLE THE BARS
Spread the peach filling over the crust. Scatter raspberries on top and then the crumbs.
Bake in the center of the oven for 50 to 60 minutes, or until the topping is golden.
Let cool completely on a rack before cutting into 2 inch bars. It's even better if you have time to refrigerate the bars until chilled and then cut from there. Once the pie bars have cooled completely, use the handles of the parchment paper to gently lift them out of the pan and onto a cutting board. Use a knife to loosen the edges if you need to.
Notes
Store the bars in a covered container in the refrigerator until ready to munch.