Generously butter a large baking dish. Put the frozen peaches and blueberries into the dish. Mix the cornstarch into the fruit.
For the topping: Melt the butter. In a medium sized bowl combine the oats, sugar, flour, cinnamon, salt and nuts if using. Stir in the melted butter until it is all incorporated. It should be a little crumbly.
Evenly sprinkle the topping over the fruit.
Bake for 35 to 40 minutes until the topping is browned and it is bubbling around the edges. Allow to cool for at least 10 minutes before serving. Leftovers can be covered and stored in the fridge for several days.