Preheat the oven to 350F/ 175C. Lightly butter an 8" glass baking dish.
In a large bowl, combine the sliced peaches (thaw first if using frozen,) the sugar, flour, lemon juice and salt. Toss to coat everything with the sugar and flour. Then gently stir in the raspberries being careful not to break them up too much. Set aside.
In a medium bowl, combine all of the topping ingredients. Using a pastry cutter or your fingers, gently work in the cold butter until pea-sized lumps are formed.
Pour the filling mixture into the baking dish and sprinkle the topping evenly over the peach mixture. Bake until the fruit is bubbly around the edges and the topping is a light golden brown, about 40-45 minutes, rotating once halfway through cooking.
Serve warm or at room temperature with vanilla ice cream. It's also incredibly good cold straight from the fridge. Enjoy!
*If you omit the raspberries and only use peaches, you can cut back about a tablespoon of sugar if you desire.
*To prepare ahead- Make the filling and pour into the baking dish and store in the refrigerator covered with plastic. Make the topping and store in the fridge seperately. Then spread the topping on when ready to bake.
*I don't recommend freezing this, but leftovers can be stored in the fridge for about 4 days.