1. Start by preheating your oven to 350 degrees Fahrenheit (175 degrees Celsius).

2. On your cutting board, slice the peaches into thin wedges and place them in a large mixing bowl.

3. To the bowl, add the granulated sugar, lemon juice, cornstarch, 1/4 teaspoon each of the cinnamon and nutmeg. Toss the ingredients together until the peaches are thoroughly coated.

4. Add the blackberries and give everything a gentle toss.

5. Pour the fruit mixture into your baking dish, spreading it out evenly.

6. In the now-empty mixing bowl, combine the flour, brown sugar, butter, oats, and remaining cinnamon and nutmeg. Use your fingers to mix until the mixture resembles coarse crumbs.

7. Sprinkle the crumb mixture evenly over the top of the fruit in the baking dish.

8. Place the dish in your preheated oven and bake for about 45 minutes, or until the topping is golden and the fruit is bubbling.

9. Allow the dessert to cool slightly before serving.

10. Serve warm, topped with a dollop of whipped cream (optional).

11. Enjoy the gentle sweetness and heartiness that would surely delight any centaur in Narnia!