In a medium bowl, combine the flour, brown sugar, white sugar, cinnamon, salt and butter. Using a pastry blender (or clean hands), incorporate the ingredients until the mixture sticks together in small crumbles.
PREPARE THE FRUIT FILLING:
In a deep pie dish, arrange the peach slices and raspberries. Sprinkle the fruit with lemon juice, sugar and cornstarch; toss. Sprinkle the crumble topping evenly over the fruit.
Bake for at least one hour- until golden brown and the the juices start to bubble through the topping. You may need to cover the top with foil at some point to keep the crumbly topping from becoming too dark or burnt. Serve warm or at room temperature. Delicious served with a little bit of vanilla ice cream!